Changing input handling.

Now events are reported by IWidgetController's children. This reduces
code needed inside specific widgets (the same code used to be repreated
in different widgets).
Initial implementation of DefaultWidgetController.
More work on the DefaultWidgetController.
Fixed list widget, added handling of keyboard focus to DefaultWidgetController.
Fixing check box, a bit of code for text input.
Fixes related to the label widget.
More work on text input.
Backspace, delete and arrows in LineEditWidget.
JsonScreenSample demo uses anchors.
Rename some backend's methods, initial work on shortcut bar.