don't show "Play using..." for plugins (fixes #15361)

This is a rather simple approach at fixing http://trac.xbmc.org/ticket/15361 where a user tries to play a video from a plugin through UPnP's remote play functionality which is currently not possible. Therefore we shouldn't show the "Play using..." context menu item for plugins.

A better fix would be to improve the logic in CPlayerCoreFactory::GetPlayers() to simply not add UPnP remote players to the list if the item is a plugin but I'm not comfortable with changing that logic as I don't completely understand it and all of the use cases there are.