upnp: add support for controlling external player
sys: add action "switchplayer" that allows switch of forced player

This allow moving a playing item to a another player (remote upnp
for example)

It's hooked up to "t" on keyboard currently.
gui: make sure we leave fullscreen if we start a player without media
upnp: issue Stop before SetAVTransportURI to make sure it's stopped

Renderers should reject this call if not stopped according to
DLNA
upnp: do a full LoadItem from thumb loaders instead of FillLibraryArt
upnp: factor out code that build a CFileItem from a PLT_MediaObject
upnp: use standard methods to build CFileItem in renderer
Platinum: add SetNextAVTransportURI support to media renderer base
upnp: renderer support for SetNextAVTransportURI