[Confluence] added volume control via slider

Hey guys,

based on https://github.com/xbmc/xbmc/pull/764, I reworked the volume control to have a slider.
Basically this adds a new group to VideoOSD and MusicOSD that has the same look and feel like
"DialogVolumeBar" but can control the volume.

Unfortunately I do not have a touch device, so I'm not sure if this is suitable for it due to the hidden button.
So maybe someone with a touch device can test and or tell me if it is working.

Or maybe there can be a <visible>!System.Platform.IOS </visible> to avoid confusion there.

Any opinion on this?
Any things that need to be changed?

cheers,
mad-max