messageTray: improve bad-markup handling

_fixMarkup() was supposed to be ensuring that the markup we passed to
clutter was correct, but it was validating the syntax incorrectly, and
wasn't checking that the markup was valid (or even well-formed). This
is bad because if you pass bad pango markup to
clutter_text_set_markup(), it will g_warn and drop the string on the
floor.

Fix by fixing up the regexps, and then calling Pango.parse_markup() on
the result, and just xml-escaping everything if parse_markup() fails.

https://bugzilla.gnome.org/show_bug.cgi?id=650298