[webui] fix status icon updates when a recording is scheduled/started from the
EPG grid

We can listen on dvrdb comet updates but we should only act on them when the
EPG grid is visible. When it's not visible the update is delayed until the
tab becomes visible again.
[webui] remove some unused stuff
[webui] render a progress bar for currently running programmes
mpegts tables: moved locking from inside to outside mux open/close