Enhance TS demuxer to support VDR edits

I use vlc to stream mpeg2 transport streams (dvb-t) via ftp. One of the things which is annoying is that the skip forward/backward hotkeys don't work.

Looking at the code I can see that this is because it can be difficult to determine the "time index" accurately. However for this use case, a reasonable approximation will do, and this can be done by estimating the mux rate based on the embedded PCR.

The attached patch (against 1.0-bugfix) does this and now the hotkeys work perfectly. It has the added advantage of now displaying the length of the show too.