TTML: allow .xml extension

Since some time VLC supports TTML subtitles. Currently the file extension .ttml is supported for such files. However also the extension .xml is widely used for this files (as it is an XML format), but currently unsupported by VLC.

Therefore I propose to add the .xml file extension for subtitles as well.

There has been an attempt in 2015 to do this (#15726 (closed)), which has been declined. However as far as I can see, the detection does not only consider the file extension but (in case of TTML) also checks for the expected namespace (see OpenDemux method in /modules/demux/ttml.c). So if I don't oversee anything it should be no issue to also add .xml here.