Make all file IO searchpath-agnostic

[ThisAMJ]

The Source Engine supports arbitrarily many "game paths" with sequential read priorities (i.e. read from first path, if non-existent read from next). A gripe I have with SAR is that certain files have to be in certain game directories e.g. tas folder in Portal 2, crosshair in portal2, etc, making the use of these features rather unintuitive and opaque without a tutorial/wiki. So, I believe that in Wormhole, all file IO should be done through the API, with support for the use of any search path so that such confusion is avoided. I've started trying to do this on SAR's download_file branch, with pretty good results.

See also: SAR FileSystem::GetSearchPaths()

Alternatively, we could create a single centralized 'common dir' for all Wormhole files (~/.config/wormhole/? %appdata%\wormhole?) and read and write to there. The question is then of course how, and how much, do we move stuff from the game folder there?

e.g. loading wormhole from .vdf may work from 'common dir'? Begs the question of ease of installation

// wormhole.vdf or smn
"Plugin"
{
    "file" "<path>/<to>/wormhole"
}

Just spitballin