Drag and drop support for maps, mods, savegames and demos

[temx]

This is a possible fix for #548. It only supports stuff from the normal search path.

[Novum]

I'm kind of on the fence about this one. Kind of odd to do drag and drop and then you can't actually drag in any file?

Maybe we should implement some menus? Just not sure where they would go.

[temx]

Yeah, I'm not so sure either.

I also considered adding this for commandline parameters but that's even worse: the application starts in the dragged file's directory, so if you want to support "drag into the exe" then you need to check for that and change dirs.

Supporting external loose files is somewhat awkward - I guess you can try to load them with the currently running mod and hope for the best. Maps come in .bsp+.lit(+.ent) pairs, so that also needs handling.

For menus there's the added problem of having to match the font / bailing out for any mods that modify it. JoeQuake for example puts everything in the main menu and forces it to keep the original style. Perhaps an inconspicuous place would be the corner of the screen?