JS timeout on Volume change

See discussion on https://github.com/volumio/Volumio-WebUI/pull/23

I would personaly like to keep having a feedback when changing the knob before releasing


Behaviour is:
- on change (sliding the knob) => volume is updated every 200ms
- on release => the volume is instantly updated

Changes are also made to make the volume knob smoother, removing inappropriate refreshs when moving the knob