playbin2: fix subtitle only seeks when switching to external subs

playbin2: fix subtitle only seeks when switching to external subs

Sending a non-flushing seek might not be enough for switching
to an external sub that has already been used because the flushes
are needed to reset the state of its decodebin's queue.

For example, if the subtitle is short enough, the queue might get
and EOS and keep its 'unexpected' return state. If the user switches
to another subtitle and back to the external one, the buffers
won't get past the queue.

This patch fixes this by adding the flush flag to the seek and
preventing that this flush leaves the suburidecodebin.

https://bugzilla.gnome.org/show_bug.cgi?id=638168

Conflicts:

	gst/playback/gstplaybin2.c