Add engine copyright header to GUI unit tests
Misc tid bits

* Text: Implement ctor for font_type::raw_ptr.
* UIWidget: NULL check for widget before insertion as a child.
* UIWidget: Make the default name tag for UIWidget to "parent_window";
was "window".
Refactoring of MessageBox & QMessageBox widgets

* MessageBox: Remove default (void) constructor.
* MessageBox: Remove operator = method overload.
* MessageBox: Set minimum size.
* MesageBox: Phase out the Text object setters, in favor of the more
robust internal construction of Text objects from variable arguments --
alike what we had developed in the branch 'feature/FontResource').
* MesageBox: Initial refactoring of events code to match recent work
done in other widgets.

* QuestionDialogBox: Remove default (void) constructor.
* QuestionDialogBox: Remove operator = method overload.
* QuestionDialogBox: Initial refactoring of events code to match recent
work done in other widgets.
* QuestionDialogBox: Introduce append_choice setter for text strings.
* QuestionDialogBox: Initial refactoring of the internal updating
and handling of Text objects (should be done more like ListBox).
Break examples/gui.cpp into multiple files