Add hb_video_quality_get_limits and hb_video_quality_get_name to libhb API.

Includes a MacGui implementation.
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 (Activity Log and MediaInfo)

- optimize the keyframe interval for Quick Sync Video (see QSV branch)
-> also, store it in a 'settings' struct in preparation for advanced option support
VT: add a few more configurable settings (hardcoded to default values for now).
VT: add H.264 profile and level support.

Can only be used via the CLI for now.
VT: only enabled under Mac OS X.

Also change the CLI name to something less generic.
VT: disable two-pass where applicable.
VT: allow using the iPod 5G atom.
MacGui: set the "Theora" options label based on the actual encoder name.