baseparse: do seek I/O in streaming thread and post progress messages for seek
Submitted by Tim Müller
+++ This bug was initially created as a clone of Bug 613351 +++
BaseParse (and other parsers/demuxers) should not do any I/O in the seek handler, as currently done in gst_base_parse_locate_time(). This should be
done from the streaming thread instead, and the element should maybe also post PROGRESS messages on the bus about seek started/finished/failed.