libobs: Add flag to force source audio to mono

This flag is actually useful under a number of circumstances, and has
been requested a number of times.
libobs: Remove inline on a function

The copy_audio_data function really shouldn't be inlined because it's
being called twice.  It's somewhat unnecessary, I think I left it inline
by accident.
UI: Add advanced audio properties dialog

This dialog gives options such as increasing audio past 100%, forcing
the audio of a source to mono, and setting the audio sync offset of a
source (which was an oft-requested feature)