docs: add missing files for distribution

 * add some documentation files in docs/design
 * add docs/list-ulink.xsl so check in docs/manual works

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=719814
baseparse: refactor pending events pushing

Refactor code repeated 3 times to a common function

https://bugzilla.gnome.org/show_bug.cgi?id=721350
baseparse: push pending events before GAP event

A GAP event is handled as an empty buffer by sinks and they expect
to receive start up events before GAP events (like a segment).

This is important specially if there is a GAP at the beginning of
a stream (before any buffers) so that the segment event can be
pushed downstream before the GAP

https://bugzilla.gnome.org/show_bug.cgi?id=721350
baseparse: remove pending_segment as it was being misused

It wasn't required, instead baseparse was using it to check the media
caps to identify if it was handling audio or video.

The pending_segment was removed and a checked_media boolean
replaced it for a more accurate naming.

https://bugzilla.gnome.org/show_bug.cgi?id=721350