UI: Allow installing texture packs from zips

[unknownbrackets]

Related, this also allows using region-specific texture ini content.

There's no UI to select a game ID to install for. So the best instructions to give users would be:

To install the texture pack:

1. Load the game first, then pause and return to the menu.
2. Now select the zip (or drag into PPSSPP on desktop) and install.
3. Profit?

My theory is that most users will only have one region anyway. The multi-region use case probably means a user who will be able to figure out unzipping it themselves. Otherwise, their preferred recent is likely to be the most recent region they have played.

For texture pack authors, all you need to do is add the following to textures.ini:

[games]
ULES12345 = textures.ini
ULJS12345 = textures.ini
ULUS12345 = textures.ini

If you use something other than "textures.ini", it will load that file in addition to textures.ini. It uses exactly the same format. Make sure the filename is lowercase so it works on all platforms.

Fixes #11737.

-[Unknown]

[unknownbrackets]

Also, for any authors looking to develop multi-region texture packs, you can use "junctions" or symlinks. For example:

cd ppsspp/memstick/PSP/TEXTURES
mklink /j ULJS12345 ULES12345
mklink /j ULUS12345 ULES12345

Would make the other two game IDs refer to the ULES12345 folder. This way you wouldn't need copies and could keep your work centralized in one folder. If you have gone to the trouble to buy the game in each region to perfect your texture pack, this should be the easier way to test it.

-[Unknown]

[hrydgard]

Some nice cleanup in here too. Good stuff.