Implement Grid widget
Fix errornous spacing in Box widgets
Improve consistency in event naming
Provide event on Enter key and access to text in Edit
Remove debug output
Implement ModalWindowLayer

Used to implement modal windows; it catches all events except those to
its child window.
Provide more means to access the current ListBox state
Fix nested focus
Provide means to actively set focus on a given widget
Extend Window class to deal with modality
Properly bubble-up mouse click events
Means to deal with modality in RootWidget
Merge branch 'ui-widgets' into devel