Add 'create new translation' button to editor metabox

[apermo]

Description

Add a "+" button to the editor metabox (both select and autocomplete modes) that allows creating a new translation directly from the post editor.

Behavior

• The "+" button is visible for each language when no post is selected/linked for that language
• Clicking opens post-new.php?msls_id=NNN&msls_lang=XX in a new tab (same URL pattern as the listing column "+")
• The button requires the current post to be saved first (not visible for auto-draft posts)
• In autocomplete mode, the "+" hides dynamically when a post is selected and reappears when the field is cleared

Context

Currently, users must navigate away from the editor to the posts listing to use the "create new translation" flow. Adding this button to the metabox provides a more streamlined workflow.