Added a clickable class & signal and slots implementation.
Added Hoverable & Focusable (Tab) Widgets.

Documentation follows.
Added documentation to the Focusable widget.
Added documentation to the Hoverable widget.
Added documentation to the Clickable widget.
Added an onPressed function to the Clickable widget.
Added return-activated press & release signals to the clickable widget.
Added missing documentation & made the hoverable event code cleaner.
Added virtual fucntions for children to detect signals directly.