Export to Matroska hangs when subtitles are selected

[fengalin]

It seems like subtitle handling is now (re)activated in the Matroska mux element, but this causes the application to hang.

[fengalin]

It might as well be related to the playbin element since there doesn't seem to be any recent changes to matroskamux WRT subtitles.

[fengalin]

Still applicable for GStreamer git master as of 20190626.