add optional marker menu

[danielroedl]

A custom right click menu can be added to a marker with

myMenu = {
  # key is the title and value is the function to call
  'My fancy marker menu entry 1': function1,
  # separators can also be added
  '-': None,
  'And a second entry after the separator': function2
}
map_widget.set_marker(x, y, menu=myMenu)

[danielroedl]

Maybe it is a good idea to add this menu option also to CanvasPath and CanvasPolygon too. What do you think?

EDIT: Done