[pvr] remove a lot of duplicated functionality from the channel list
context menu. Some of the functionality is already present in the
channel manager dialog, and for consistency some options (filter and
show hidden channels) have been moved to the side blade (similar to
how it's done for e.g. movies).

The consistency between the context menus for movies and TV channels is
further improved by adding a "Manage" item. To preserve the
functionality "lost" when a lot of the context menu items were removed,
"Manage" contains a shortcut to the channel manager.
[confluence] remove height limitation from the PVR side blade
[confluence] move the "group items" radio button under "Misc Options"
[language] rename "Group management" to "Group manager" to better match
"Channel manager"
[settings] make "Group manager" accessible from the Live TV settings,
just like the channel manager
[confluence] bump version to 2.6.0
[gui] bump xbmc.gui to 5.6.0
[squash] inject parent implemenation's context buttons before the Manage
button