Add RmlUi UI framework integration for SpringBoard dialogs

Implement RmlUi-based UI system as a modern replacement for the legacy
Chili framework. This initial conversion focuses on dialog components as
they are self-contained and easier to migrate independently.

New Infrastructure:
- RmlUi Manager (scen_edit/view_rmlui/rmlui_manager.lua)
  - Context creation and lifecycle management
  - Document loading and event routing
  - Input handling (mouse, keyboard)
  - Screen resize and rendering coordination

- RmlUi Widget (LuaUI/widgets/api_sb_rmlui.lua)
  - Spring engine integration
  - Drawing and input event forwarding
  - Layer -999 for proper rendering order

Dialog Conversions:
- Base Dialog class with common functionality
  - Event binding (OK, Cancel, ESC, Enter)
  - Field value management
  - Error message display
  - Lifecycle management (Show/Hide/Close)

- NewProjectDialog - Create new SpringBoard projects
  - Project name input and validation
  - Map selection with dynamic field visibility
  - Blank map size configuration

- FileDialog - Base file browser dialog
  - File/directory listing
  - Single/double-click selection
  - File type filtering support

- SaveProjectDialog - Save project dialog
- OpenProjectDialog - Open project dialog with validation

Templates & Styling:
- RML templates (scen_edit/view_rmlui/rml/)
  - base_dialog.rml - Generic dialog template
  - new_project_dialog.rml - New project UI
  - file_dialog.rml - File browser UI

- RCSS stylesheet (scen_edit/view_rmlui/rcss/dialog.rcss)
  - Dialog window styling
  - Form field styles
  - Button styles with hover states
  - Dark theme matching Chili appearance

Documentation:
- RMLUI_CONVERSION.md - Comprehensive conversion guide
  - Architecture overview
  - Migration checklist
  - API reference
  - Testing instructions

- scen_edit/view_rmlui/README.md - Technical documentation
  - Directory structure
  - Component mapping
  - Usage examples
  - Known limitations

Requirements:
- BAR Engine 105.1.1+ or Recoil Engine (RmlUi not in standard Spring 104.x)
- Chili UI framework remains for non-converted components

Status: Phase 1 (Dialogs) complete, testing pending with BAR Engine

Author: claude

LuaUI/widgets/api_sb_rmlui.lua

@@ -0,0 +1,142 @@
+function widget:GetInfo()
+    return {
+        name      = "RmlUi Framework (SpringBoard)",
+        desc      = "RmlUi UI framework integration for SpringBoard",
+        author    = "SpringBoard Team",
+        date      = "2025",
+        license   = "GPL",
+        layer     = -999, -- Draw before other widgets
+        enabled   = true
+    }
+end
+
+-- RmlUi integration for SpringBoard
+-- This widget provides RmlUi rendering and input handling
+
+local rmlui
+local initialized = false
+
+function widget:Initialize()
+    -- Check if RmlUi is available
+    if not Spring.RmlUi then
+        Spring.Log("SpringBoard RmlUi", LOG.WARNING, "RmlUi not available in this Spring engine version")
+        widgetHandler:RemoveWidget(self)
+        return false
+    end
+
+    rmlui = Spring.RmlUi
+    initialized = true
+
+    Spring.Log("SpringBoard RmlUi", LOG.NOTICE, "RmlUi Framework initialized")
+    return true
+end
+
+function widget:Shutdown()
+    initialized = false
+end
+
+function widget:ViewResize(vsx, vsy)
+    if not initialized then
+        return
+    end
+
+    -- Update RmlUi context dimensions
+    if SB and SB.rmlui then
+        SB.rmlui:UpdateDimensions()
+    end
+end
+
+function widget:DrawScreen()
+    if not initialized then
+        return
+    end
+
+    -- Render RmlUi context
+    if SB and SB.rmlui then
+        SB.rmlui:Render()
+    end
+end
+
+function widget:MouseMove(x, y, dx, dy, button)
+    if not initialized then
+        return false
+    end
+
+    if SB and SB.rmlui then
+        return SB.rmlui:MouseMove(x, y, dx, dy)
+    end
+
+    return false
+end
+
+function widget:MousePress(x, y, button)
+    if not initialized then
+        return false
+    end
+
+    if SB and SB.rmlui then
+        return SB.rmlui:MouseButton(x, y, button, true)
+    end
+
+    return false
+end
+
+function widget:MouseRelease(x, y, button)
+    if not initialized then
+        return false
+    end
+
+    if SB and SB.rmlui then
+        return SB.rmlui:MouseButton(x, y, button, false)
+    end
+
+    return false
+end
+
+function widget:MouseWheel(up, value)
+    if not initialized then
+        return false
+    end
+
+    -- RmlUi mouse wheel handling would go here
+    -- Implementation depends on RmlUi API availability
+
+    return false
+end
+
+function widget:KeyPress(key, mods, isRepeat, label, unicode)
+    if not initialized then
+        return false
+    end
+
+    if SB and SB.rmlui then
+        local handled = SB.rmlui:KeyEvent(key, true, mods)
+        if handled then
+            return true
+        end
+
+        -- Handle text input
+        if unicode and unicode > 0 then
+            return SB.rmlui:TextInput(unicode)
+        end
+    end
+
+    return false
+end
+
+function widget:KeyRelease(key, mods, label, unicode)
+    if not initialized then
+        return false
+    end
+
+    if SB and SB.rmlui then
+        return SB.rmlui:KeyEvent(key, false, mods)
+    end
+
+    return false
+end
+
+-- Expose RmlUi to other widgets
+function widget:GetRmlUi()
+    return rmlui
+end