[pvr] introduce a flag for channels which indicates whether
the user has changed the channel name
[pvr] indicate that the channel name has been manually changed
when the user changes it from the channel manager
[pvr] clear the userSetName flag if the user changes the name to
an empty string

This way the channel name will be updated from the client once
XBMC is restarted
[pvr] update channel names from the client unless it has been manually
changed by the user (fixes #14930)
[pvr] clear the "user-defined name" flag and fetch the channel name from the
client if the user saves a channel with an empty name