Add a volume control

[leolost2605]

Fixes #51

Allows adjusting volume of currently playing video.

Because of the action bar the space on the left looks a bit weird currently but that's fixed with #362

[danirabbit]

I'm kind of more inclined to go the route of per-app volume controls in the sound indicator/settings so that we're not duplicating sound UI everywhere. Having sound controls in the app means a situation where you may be unable to hear audio even though controls are showing full volume. Having all the volume controls together would make sure it's obvious when the total system sound is lowered and we can be smarter about maybe only showing several volume controls when several sound sources are available etc

[leolost2605]

For some reason the gstreamer reported volume is actually synchronized with the pulseaudio per application volume in the settings, no idea why though 😅
It might be useful to have a more convenient way to access the same setting for this app here but we can keep it without too (although then I would really like to have the setting accessible through the indicator).

[ryonakano]

@leolost2605 @danirabbit Maybe we can close this in favor of per-app volume control in elementary/settings-sound#253?