[pvr] fix: set flag UserSetIcon if channel icon is changed in pvr channel manager

If an user set its own icon for a channel, this one is reset to null upon xbmc restart because UserSetIcon flag is not set.
[pvr] fix: non-existing icons will never be delete from the database
[pvr] use also custom channel name to search for channel icon

If an user set a custom channel name for a pvr channel, this name should
also be used when searching for an icon.
Merge pull request #4477 from xhaggi/pvr-icon-fixes

[pvr] fixes related to chanel icons