Text: Copy assignment crash fix

* This corrects a crash that occurs when nom::Window copy assignment
operator is called.
TrueTypeFont: Change rendering of text from Blended to Solid
Refactoring of Button class

* Rename GrayWindow to GrayFrame

* Introduce Button base class with label text functionality and
relocate bitmap functionality from original Button into new
BitmapButton class (of which inherits from Button).

* Introduce initial implementation of Button::on_event and
BitmapButton::on_event method logic.

  examples/gui:
* (Re)-add create_button_* tests.