gst/: Put more strings into the GLib quark table. No need to keep a hundred-something copies of identical version str...

Original commit message from CVS:
* gst/gstplugin.c:
* gst/gstplugin.h:
* gst/gstregistrybinary.c:
* gst/gstregistryxml.c:
Put more strings into the GLib quark table. No need to keep
a hundred-something copies of identical version strings,
license strings, package name strings and package origin
strings around.