Map browser, used in the gamesetup and in the main menu.
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