Map/gametype vote layout changes

Improvements over the map/gametype vote screens:
* items as big as possible with minimum alteration of the desired item aspect ratio (like in the weapons panel layout)
* layout more compact with limited item size (especially with very few vote choices)
* added a background in the default hud skin, as part of the change that makes them real panels
* fixed a few minor things
* the fps panel (if enabled) is kept on screen during the map vote


See merge request !152