Remove unused fonts

* Our system fonts cache obsoletes the need to hold onto these, since
we depend on the being able to find the font on the end-user's system.
(Experimental) Refactor widget's events processing

* ListBox: Remove default (void) constructor.
* ListBox: Refactor events processing; move the generic bits of code
to UIWidget::process_event, and tie in the rest to virtual on event
methods, provided & called by its base class (UIWidget).

* Button: Refactor events processing; move the generic bits of code
to UIWidget::process_event, and tie in the rest to virtual on event
methods, provided & called by its base class (UIWidget).

* BitmapButton: Refactor events processing; move the generic bits of code
to UIWidget::process_event, and tie in the rest to virtual on event
methods, provided & called by its base class (UIWidget).