Fixes in subview widget and event enumeration

  - Moved the mouse enter/leave events to canvas generated events
  - Added more comments to the subview widget code
  - Improved selection of events redirected to subview callback
Made skin pixmap and font cache creation lazy

 - Platform dependend global skin pixmap and font cache are created when
   they are required
 - If an application doesn't create a window with a native canvas, no
   native font cache and skin pixmap are created
 - Similar to OpenGL backend