Allow Canvas Windows to be resized by dragging

Allow Canvas Windows to be resized by dragging

 - Setting 'resize' property to true on canvas::Window shows
   resize icons and exposes requested size to the property
   tree. This can be used eg. from Nasal to actually resize
   the window and/or show a preview of the resized window
   while resizing.
 - Event handling now ignores events which have already
   been handled. Before eg. clicking inside a window
   also caused picking to be performed inside the
   scene which is for sure not the expected behaviour.
 - Also forwards scroll wheel events from canvas::Window.