The threading logic was changed somewhat. UBQuickTimeFile's run()
function no longer handles enqueuing the video/audio samples in a while
loop. Instead, it runs once, and uses Apple's Dispatch Queues to handle
enqueuing samples.
One dispatch queue was thus added for each input to the AssetWriter.
Each input is associated to one queue, and the requestMediaDataWhenReady
function insures that the inputs go and fetch any available samples when
they are able to write them.

As tested (for short podcasts, repeatedly), this solves all the problems
encountered earlier, such as the program hanging due to one input not
being ready, or corrupt files due (presumably) to missing samples.

clean-up; temporary solution for waiting until audio assetWriter is ready before appending sample buffers

Forgot to remove a couple of things when cleaning up the code for the
previous commit

Re-coded this feature (which had been temporarily removed due to the
old code being reliant on the obsolete QuickTime library).

Migrated all QuickTime-related code to modern AVFoundation / Core Video
/ Core Media equivalents.
Audio support was temporarily removed; to be re-established ASAP.

Beginnings of doxygen-style function documentation was added

Q_WS_MACX -> Q_OS_OSX
Q_WS_MAC -> Q_OS_OSX
Q_WS_WIN -> Q_OS_WIN
Q_WS_X11 -> Q_OS_LINUX