libobs: Fix signal handler lock bug

There was a return statement within a section of code that had a locked
mutex.
libobs: Add level/magnitude/peak volume levels

This replaces the older code which simply queried the max volume level
value for any given audio.

I'm still not 100% sure on if this is how I want to approach the
problem, particularly, whether this should be done in obs_source or in
audio_line, but it can always be moved later if needed.

This uses the calculations by the awesome Bill Hamilton that OBS1 used
for its volume levels.  It calculates the current max (level),
magnitude, and current peak.  This data then can be used to create
awesome volume meter controls later on.

NOTE: Will probably need optimization, does one float at a time right
now.

Also, change some of the naming conventions.  I actually need to change
a lot of the naming conventions in general so that all words are
separated by underscores.  Kind of a bad practice there on my part.
libobs/media-io: Remove obsolete mixing functions

Also, Remove the volume level processing from audio-io.c, it was moved
to obs_source instead.
UI: Add new volume meter handling

This adds support for the more smooth volume levels that accounts for
both level and magnitude.  Currently, it just averages level and
magnitude, later on a full audio meter control can be made that would
properly utilize level, magnitude, and peak.

Also cleaned up the code a bit and removed some trailing whitespace.