Splash + User Feedback + FPS

Large number of changes in this commit:
- On-screen FPS and Frame Timing counters - Push SHIFT+F or F9
- Splash Screen - Visible for 10 seconds at start of game, includes
version number
- Added HMD stats (HMD Type / orientation / position / VRBoost) - Push
SHIFT+S or F10
- Toast Notification when toggling functions (turn Positional tracking
on and off to see, work for all toggles)
- Notifications on error situations (no HMD, user outside of positional
tracking zone, VRBoost failure)
- DK2 users will be asked to "calibrate" the HMD for positional tracking
once the splash screen has been dismissed
- Previous changes broke tracking for other trackers, should now be
fixed (though have no way to verify)