This grid-based system allows browsing all available maps at a glance,
encouraging more diversity and making it nicer to pick a map.
Moves the mapCache and the map filters controller to the gui maps/
folders, and include that folder where it is used, instead of them being
in common/ or the gamesetup.
Comments By: Freagarach
Patch By: Nani (reworked by elexis then wraitii)
Fixes#5315 (though further work, such as proper scrolling, would be
nice).
Differential Revision: https://code.wildfiregames.com/D1703
This was SVN commit r24459.
