VT: set the "Theora" options label based on the actual encoder name.
VT: rate control improvements:

- implement a 'fake' Constant Quality mode (automatic bitrate selection)
-> not optimal, but allows for "set and forget" usage to some extent

- set the average bitrate correctly
-> 1 Kbps = 1000 bps, which is confirmed by testing (checking the log and Media Info)

- optimize the keyframe interval for Quick Sync Video (see QSV branch)
-> also, store it in a 'settings' struct in preparation for advanced option support