Added GUI for RTSP sources

Thanks for your work on RTSP sources, it is highly appreciated!

I've added a separate tab in the UI where RTSP sources are edited. The sources are still stored in the same way inside the tvheadend config, but filters are added so that multicast sources are displayed in the IPTV tab and the RTSP sources are displayed in a new "RTSP sources" tab.

Also, I've merged in the latest changes from the tvheadend/tvheadend master to bring the branch up to date.

I hope that these changes will make it easier for the tvheadend team to accept your rtsp changes.