docs: update links to developer.gnome.org

The URL layout has changed. Fix the links and comment out one paragraph where
the doc is gone.
Fixes #702135
docs: change https to http urls

Thank you browser for needlessly changing to https for static doc pages.
docs: check for broken links in docs

The check is done using curl (if available). It lists the curl exit code + http
status code (for those > 399) together with the use of the url in the code. The
check is not fatal.
docs: fix some external links
basesink: call state change in all cases

When we asynchronously go from READY to PLAYING, also call the
state change function so that subclasses can update their state for PLAYING.
Because the PREROLL lock is not recursive, we can't make this without
races and we must assume for now that the subclass can handle concurrent calls
to PAUSED->PLAYING and PLAYING->PAUSED. We can make this assumption because not
many elements actually do something in those state changes and the ones that
did would be broken even more without this change.

https://bugzilla.gnome.org/show_bug.cgi?id=702282
tests: add stress test for buffers and pools
docs: add missing file for doc-link check
buffer: Fix wrong size/index handling when merging memory

https://bugzilla.gnome.org/show_bug.cgi?id=702617
buffer: Add unit test for map_range()

https://bugzilla.gnome.org/show_bug.cgi?id=702617
queue: Don't hold the queue mutex while doing serialized queries downstream

https://bugzilla.gnome.org/show_bug.cgi?id=702520
taglist: Avoid combinatorial explosion when merging tags

When appending/prepending tags, avoid re-creating (and copying) lists if we already
have one and instead just append/prepend the GValue to the list.

https://bugzilla.gnome.org/show_bug.cgi?id=702545
basetransform: optimize default acceptcaps implementation

Pass the fixed caps we're asked to accept as a filter for the caps
query, so we don't get a fully-expanded set of caps back (which we don't
need and can take a lot of time for intersection).

This reduces the time for camerabin to produce a second frame on a
logitech C910 camera from around 52 seconds to a bit less then 16
seconds on my system.

https://bugzilla.gnome.org/show_bug.cgi?id=702632
pad: Add a filter to the caps_query done by acceptcaps

Use the caps that the pad is asked to accept as filter for the query

https://bugzilla.gnome.org/show_bug.cgi?id=702632