libRocketTest: Move DataGridStore tests to its own files

* Additional prototyping of DataGrid modeling for TTcards (paging, event listeners, etc.)
RocketSDL2SystemInterface: Use LOG_CRIT & return false on LT_ASSERT

* Use NOM_LOG_CRIT for LT_ASSERT log messages; the default logging
priorities would exclude assert messages logged with NOM_LOG_DEBUG,
so this *should* enable logging of LT_ASSERT messages from libRocket.

* Use boolean return values properly in ::LogMessage interface to
signal to libRocket whether or not to stop execution (presumed to be
applicable only to its internal code).
Update third-party/README.md

* Include build notes for libRocket on OS X.
libRocketDataGridTest: Fix sometimes-slow init of CardsMenu class
UIQuestionDialogBoxTest: Add second question dialog widget

* This is to help test potential integration issues with widget focus
when multiple widgets are in use (on the same context).
Fix documentation regarding mouse wheel X & Y values

The documentation for EventHandler::on_mouse_wheel is wrong -- the
values are inverted from what they should be.
UIContextEventHandler: Add event translation logic for X wheel values
libRocketDataGridTest: Prototype keydown && mouse wheel for CardsMenu
libRocketDataGridTest: More refactoring

* Remove unnecessary functions (i.e.: load_window, std::vector of docs,
etc).
* Callbacks are now free-standing functions.
* Rename CardsMenu to UIDataViewList, inherit from UIWidget && rename
unit test.
* Initial breakup of modeling functionality from CardsMenu into
CardsMenuModel; UIDataViewList uses an object pointer to this model
interface.