GtkBuildableParser: Add a wrapper for GMarkupParser

GtkBuildableParser: Add a wrapper for GMarkupParser

This currenly just wraps GMarkupParser, but the plan is to expose this
instead of GMarkup in the GtkBuildable interfaces, allowing us to
replace the parser with something that handles pre-parsed input
instead.

Note that we duplicate some of the features of GMarkup to implement
the APIs rather then call down to GMarkup, as we need to support these
in the pre-parsed case anyway.