CMake cleanups and additions

Moves the list of possible OBS specific search locations from inside every single find module into the ObsHelper, which sets it once for all modules.

Adds a backported FindThreads module, so libobs is propperly built with pthreads enabled on linux/unix.

Replace libav* find modules with a single FindFFMpeg module with components for each library.

Add helper function to install plain datafiles somewhere else than into plugin data dirs.
Not used anywhere, but could be handy.

Export the plugin data destination via CMake, so external plugins using the LibObsConfig know where to put stuff.