[pvr] Improve PVR channel context menu

This started out pretty small scale but grew a bit once I started working on it, sorry about that. The issues this PR tries to fix are:

* the context menu for PVR channels is absolutely massive. A lot of the things available through it are already implemented in the channel manager dialog, which until now has been accessible only by going all the way to the settings window. This also meant that the functionality has been duplicated in `CGUIWindowPVRChannels`.
* some functionality is expected to be in the side blade but has been implemented as context menu actions instead

This PR does the following:

* removes the "Add to favourites" context menu item for PVR channels. I don't even know what this is supposed to do. The "Switch to channel" menu item has been removed too since if you don't have a way of pressing "Select" you're screwed anyway.
* replaces the functionality duplicated from the channel manager and adds a "Manage" item which contains links to the group and channel manager, as well as the "Update EPG information" item.
* moves "Show hidden channels" and the filtering functionality to the side blade
* makes the group manager accessible through settings as well

@xhaggi @opdenkamp for review.