Add support for loading JKA assets.

[Daggolin]

Add support for loading JKA assets:

• cvar fs_assetsPathJKA; works similar to fs_assetsPath, but automatically tries to detect jka rather than jk2 (using registry keys on Windows and known paths on Mac)
• new cvar fs_noJKA; disables loading of jka assets
• jka assets are loaded before jk2 assets, thus jk2 files take precedence in case of conflicts
• .shader files from jka assets are virtually renamed to .shader_jka and .shader_jka files are loaded with lower priority than .shader files
• when trying to load an image the renderer tries to load all extensions without considering the jka assets and only includes jka assets if all extensions failed; this avoids cases of a higher priority extension images from the jka assets overriding jk2 images

Known (non-)issues:

• as jka has intentionally been given a lower priority there can be cases where jka maps (or models) use jk2 versions of textures or shaders, making them look different than intended; this has already been the case before jka content was even available, making this irrelevant for this change
• baseja supports entities that basejk doesn't support, as well as ICARUS scripting; jka maps may play different on jk2; adjusting this would be up to mods

To consider:

• fs_noJKA defaults to "0", automatically enabling jka assets if available
• fs_noJKA is currently not exposed on the menu; I think adding it to the menu would be a good idea, but we can do this after merging the branch so this can get some testing on development builds

Solves #80.

[aufau]

I'd like to see list of all base JKA asset types and give each type a thought. do you have such list? Maybe even use a whitelist when loading. You had to do some processing with .shader files, we may add some more stuff like this in the future.

It may be interesting to, for example, let player choose if he wants to use jka or jk2 player model. Same with model skins (harder).

I think trap_FS_GetFileList() returning JKA assets may mess up some mods, but it's not a deal breaker. There is always an option to use fs_noJKA.