libobs: Add volume meter object

This adds a volume meter object to libobs that can be used by the GUI
or plugins to convert the raw audio level data from sources to values
that can easily be used to display the audio data.
The volume meter object will use the same mapping functions as the
fader object to map dB levels to a scale.
libobs: Add IEC 60-268-18 compliant fader type

This adds a new fader type that implements a position/dB mapping
as specified in IEC 60-268-18.
libobs: Add Logarithmic fader type

This adds the conversion function used by the old obs based on a
logarithmic scale.
obs/libobs: Use new volume meter api

This replaces the old code for the audio meter that was using
calculations in two different places with the new audio meter api.
The source signal will now emit simple levels instead of dB values,
in order to avoid dB conversion and calculation in the source.
The GUI on the other hand now expects simple position values from
the volume meter api with no knowledge about dB calculus either.
That way all code that handles those conversions is in one place,
with the option to easily add new mappings that can be used
everywhere.
Merge pull request #306 from fryshorts/volume-meter-improvements

Volume meter improvements