Volume meter improvements

This patchset creates a new volume meter api in libobs to move all code that deals with audio volume calculations into one place.
This will change the source api to signal simple audio levels instead of dB values and likewise change the volume meter in the gui to receive position/deflection values.

This also adds two new fader/volume meter mappings: IEC 60-268-18 and the good old logarithmic from obs 1.
The different mappings are displayed in this chart:
![](http://i.imgur.com/g3Pq2Tp.png)