Cleaning up the custom paint tool cursors

[Eben]

I like the use of the custom shapes for various paint tools for indicating the selected tool, however they aren't actually that useful as cursors.

These should instead be implemented as modified cursors, which "badge" the cursor with the shape or tool icon but retain the pointing head of the arrow. This will provide a more seamless experience and allow for a bit more precision with the tools.

Please see "gtk_drag_get_cursor" for examples. The summary of how to do this is to retrieve the graphic for "left_cursor_modify" from the theme, composite the image for the tool, and then set the cursor.

[godiard]

Fixed in 28