Trenchbroom: Add the ability to browse to loose textures location via file browser

[HeadClot]

Adding the ability to set where the textures folder is would be really helpful for newbies who are just getting started with trenchbroom.

The work flow would be like.
Click New Map -> Open Preferences -> Generic Game Config -> Then select the path for the textures folder.

[kduske]

related to #2957

[etc0de]

This would also make the not-100%-obvious knowledge about the hardcoded "textures" folder that led me to confusion & filing #2901 unnecessary, since one could simply add a location in the Generic Game Config. (I did check there as a user, except right now it's not immediately obvious how the game path relates to the texture path which would be resolved if it was possible to put them separately)

[Danimaltl]

It seems like for a generic config, texture directories could be loaded in the same fashion as files. Allowing you to choose the four path types. The paths end up in the map file in almost exactly the same way already, and you could choose whichever one works best for your compile tools. This would make it simple for those who just want to get textures into trenchbroom without being tied to a game's pattern.

However, since the pattern of a textures directory is so common maybe the interface should remain the same in those configs. If so, the texture collections menu should show what directory each textures dir is a child of:
<ModDir>\textures
baseq2\textures
baseq2\[pak0]\textures

Hmm, maybe I'll make an issue just for that bit... Anyway, there's a lot of design, issues, and complexity up in the air for new texture loading so I'm curious if this is a reasonable take.

[etc0de]

I just realized, I think textures are supposed to be specified relative to the .mtl, not the base directory of the game. So this not only depends on where the textures are, but also on where the maps are (which might also be in a subfolder)

Maybe the path prefix should just be configurable? So I can e.g. specify ../../textures or whatever, to be prepended to outdoor_png_collection/bla.png or whatever the current texture is.

It would also be nice if this could be set in the options dialog of the generic game too rather than only via creating an entirely separate game config. That process is nice for advanced users but isn't exactly intuitive.