Improve the performance of the game setup screen on first load

[Kieran P]

Since it seems slow when I test it with an empty cache, and leads to pauses when images are being cached.

Precache them on game load? Or maybe trigger caching when user opens single/multi player setup screen?

[Philip Taylor]

The issue is loading the map XML files (in order to get their names for the list). With my currently-installed maps and an empty cache, it takes about 2 seconds to load them all, of which about a third is simply parsing the XML and the rest is converting to XMB. The XMB conversion could probably be optimised a lot but the parsing is still too slow. We should do the same as with textures: store the cached converted XMB versions in the release package so normal users won't have to load the original XML versions.

[philip]

(In [8782]) Handle XMB caching with the same system as texture caching. Fixes #694.