[Estuary][PVR][guiinfo] PVR Providers User Interface

[ksooo]

We have support for PVR providers in PVR core and PVR client add-ons API for a long time already, thanks to @phunkyfish. This PR brings this core feature to the end user, by implementing respective user interface components and Estuary support.

Screen shots:

Changes to the Skinning Engine:

• New window: PVRProviders.xml
• New graphics: DefaultPVRProviders.png, DefaultPVRProvider.png

New GUI info labels:

• ListItem.MediaProvider, VideoPlayer.MediaProvider, MusicPlayer.MediaProvider - string, containing the name of a media provider, implemented currently for PVR recordings, channels, EPG (via channels), timers (via channels)

New window IDs:

• "radioproviders"
• "tvproviders"

Runtime-tested on macOS and Android, latest xbmc master, with (pre-release) versions of several PVR Client add-ons, in mixture of add-ons supporting providers and not supporting providers (pvr.demo, pvr.hts, pvr.plutotv)

@phunkyfish can you review (and runtime-test) this? @jjd-uk can you please have a look at the Estuary changes?