This avoids a race where the timeout callback is scheduled to run but we
get sufficient information to finish discovery before actually getting
around to executing the callback. See the documentation of
g_source_is_destroyed() for more details.

https://bugzilla.gnome.org/show_bug.cgi?id=639730

This ensures that everything is properly cleaned up before the
GstDiscoverer object is freed. Specifically, it makes sure that we've
removed the async timeout callback before freeing the object to avoid a
potential crash later on.

https://bugzilla.gnome.org/show_bug.cgi?id=639755

Some cosmetic changes and expands on some bits of the documentation to
make it more newbie-friendly.

Besides the duration we can also query the seekability of a stream. Use the new
API in the gst-discoverer tool.

API: gst_discoverer_info_get_seekable

gst_discoverer_discover_uri() expects the caller to unref the returned
GstDiscovererInfo object. The corresponding gtk-doc annotation was not
updated to reflect this.

Better provide something than nothing

https://bugzilla.gnome.org/show_bug.cgi?id=632988

There's no reason to make the marshaller public API. Don't install
pbutils-marshal.h header file and use prefix that makes sure the
symbol doesn't get exported.

This was a leftover from the changes from a flag to an enum

While the doc parser allows for certain variation, it is a good idea to not
use random characters here and there, but try to stick to the little markup
syntax there is.

Fixes build on macosx

Fixes #625944