OpenVR: Check if steam VR is installed, and if not, don't attempt to start up

[Riot]

As per https://github.com/ValveSoftware/openvr/issues/59, ~/.config/openvr/openvrpaths.vrpath must exist for OpenVR to function.

First check if ~/.config/openvr exists. If not, check if ~/.openvr/ exists. If so, symlink it to ~/.config/openvr. If not, abort OpenVR initialisation gracefully before it explodes into flames.

[Riot]

runtime availability check obviates the need for this now

[Riot]

Removing milestone: Beta (automated comment)