(Experimental) refactoring of font resource management

  nom::MessageBox:
* General clean up / removal of old code.
* Break apart labels_ class var into two individual objects.
* Re-enable copy constructor & copy assignment operator methods.
* Rename title_string to title_text, text_string to message_text,
text_bounds to message_bounds.
* Rename set_title to set_title_label & set_text to set_message_label
* Introduce efficient set_title_text, set_message_text methods.

  nom:Window:
* Change argument in ::set_font method from a reference to pointer.

  nom:Text:
* Introduce minimal Text constructor
* Introduce Text constructor with text color default param
* Change argument in ::set_font method from a reference to pointer.

  examples/gui.cpp:
* Add create_listbox_ex1 method (test widget with truetype_font
objects).