Add possibility to create "Close" bitmap button from XRC

This requires refactoring NewCloseButton() in order to extract
CreateCloseButton() from it, as XRC relies on being able to use two-step
creation which was previously impossible for this kind of buttons.

CreateCloseButton() is rather unusual, as it has to be declared in the
derived, platform-specific class, in order to be able to call its
Create(), but is defined only once in common, platform-independent,
code. However the only alternative seems to be to have a static
function, e.g. InitCloseButton(), which wouldn't be very pretty neither.

Closes https://github.com/wxWidgets/wxWidgets/pull/2118

misc/schema/xrc_schema.rnc

@@ -686,6 +686,7 @@ wxBitmapButton =
         stdObjectNodeAttributes &
         stdWindowProperties &
         [xrc:p="o"] element default  {_, t_bool }* &
+        [xrc:p="o"] element close    {_, t_bool }* &
         [xrc:p="important"] element bitmap {_, t_bitmap }* &
         [xrc:p="o"] element selected {_, t_bitmap }* &
         [xrc:p="o"] element focus    {_, t_bitmap }* &