Move libRocketTest into GTest derived class
QuestionDialogBox && MouseButtonEventListener tests

* Modularize unit tests.
* Apply absolute positioning to document windows.

* Introduce experimental DecoratorPhotograph interface; internally uses
nom::Texture for loading from files.
Styling sheets clean up

* Additional settings as per TTcards theme (widths, heights, paddings,
etc.).

* Add OpenSans, OpenSans-Bold to nomlib's cache of system fonts
that are available at run-time.

* Initialize system font cache for libRocket test && append Arial,
OpenSans fonts onto libRocket's fonts list.

* Fix couple minor unti test failures (title changes).
Introduce diagonal resize cursor

* Rename rkt.css to reset.css, invader.rcss to ttcards.rcss.

* Remove un-used sample resource data files leftover from libRocket
installation.

* Introduce load_cursor helper method for unit test.