Need a volume slider

[javifernandez]

Is your feature request related to a problem? Please describe.
There is no way to control the audio volume from the UI; when using controllers using the HW buttons is not convenient.

Describe the solution you'd like
A widget to control the audio volume from Wolvic UI

Describe alternatives you've considered
The HW buttons can be used, and some websites have their own volume slider (eg YouTube)

Additional context

[svillar]

For the volume of the system you have to use the HW buttons, that's the right way to do that. And for specific media elements the players do already have volume slider. We should not duplicate the UI elements.

That said, we should probably add a slider when in fullscreen, actually I think we did have it at some point.