No precise dB indicator for Audio Bus when editing volume

[nobuyukinyuu]

Godot version:
3.1.1 stable

Issue description:
Audio buses provide a volume slider to adjust the master volume for that bus. While useful, the size of this slider doesn't allow for precise editing or even direct reading the current value of the bus volume through the UI. In general, AudioBusLayout doesn't expose any bus values in the inspector, so more precise editing functionality would have to be accessed through the AudioServer manually. One workaround would be to write an addon which handles this interaction, but a simple quality of life improvement would be to give the volume sliders in the audio tab a tooltip so users know the precise current value,