Fix a bogus warning message on the Channel Recording Priorities screen

This also changes the name of the text area to 'nochannels_warning'. The
old name 'norecordings_info' is still accepted but is depreciated.
MythMusic: in the playlist editor tree sort album tracks by track number

Also now we sort the nodes in the tree make sure the tracks get added to the
active playlist in the same order they are displayed in.

Fixes #11398.
MythMusic: hunt for tracks in the 'Music' storage group

This is just for testing and currently only looks on the master backend and
only if it wasn't found in the local music directory.
MythMusic: allow seeking when playing back from a storage group
MythMusic: re-factor the IODevice handling

This makes the various DecoderIOFactory*'s responsible for the IODevice they
create and avoids passing pointers to them around.