Update license headers
Take into account PANGO_SCACE in python-pango-interface
Add function to return logical extents from pango layout
Fix identifier-keyword-clash in EllipsizeMode
Convenient access to main screen in Application descendants
Util class which handles rendering of a label

Also handles all extraction of text-related CSS properties, thus it's a
nice abstraction for any component which has to display some text.
Properties to access CSS states
Make layers of RootWidget accessible
ScreenWidget takes into account margin of children
Lazy alignment
Fix behaviour with parent-less widgets
Add Button widget