Basic renderer drawable handling and colors.
Added DrawableContainer.
Removed obsolete renderer code. Renderer inherits from DrawableContainer.
Renamed Element::bounds to area.
Added Event.
Implemented basic event propagation.
Changed code style. Started EventPropagator.

Code style changes:

  * x() -> get_x()
  * x( value ) -> set_x( value )
Added Element::create_child.
Added ElementSelector.
Added ActionHandler.