Using unified volmeters callback instead of individual ones

[avoitenko-logitech]

Attempt to increase performance and responsiveness of UI via aggregation of volmeters data into a single chunk and single call instead of bunch individuals calls. This change actually unifies volmeters processing with sources processing.

As you may notice, I removed more code than added. Some logic was moved to back-end to make front-end life easier ;-)

PR in obs-studio-node: https://github.com/stream-labs/obs-studio-node/pull/1476