Added controlled layout widget creation, fixed missing hover action for widgets from layout

I've implemented three changes:

- Possibility of customizing screen dimensions
- Added controlled widget creation from layout. This is achieved by letting the user provide a field "order" with the widget with lowest order values being processed first and the highest processed last.
- Fixed missing hover action for items when widgets created from layout file was overlapping. E.g. the hover action will not work for a button placed ontop of a background image occupying the entire screen (both from a layout file).

See http://getmoai.com/forums/post8554.html#p8554 for some additional background and discussion regarding the missing hover action.