Multiple demodulators and related interactivity

General:
- Can now add technically unlimited demodulators, they will enable/disable as they are in range. They don't yet terminate/regenerate but the framework is there.  This seems to naturally lead through experimentation to being able to keep an entire history of demodulators permanently on the frequency line and just mute/unmute as desired.  Bookmarking/history logic flows from there.
- (temporary code) Inactive demodulators are sent a dummy IQ frame when idle to kick the queue but should probably deactivate completely until in range or re-used.
- OSX currently has an issue that the audio for additional demodulators doesn't appear to be going anywhere?  Windows 8.1 WASAPI handles it no problem.
- Demodulators are now labelled by frequency, an index or count seemed inappropriate since there's no limits.

Waterfall:
- Clicking the waterfall to focus appears to be required on OSX
- Hover logic for active and last active demodulator, indicating if one will be moved
- Demodulators can be added by holding SHIFT
- Demodulators can be moved to a new range by holding ALT
- Demodulators can be created at a new range by holding SHIFT+ALT
- Demodulators can be deleted by hovering and pressing DELETE or D