This commit keeps us moving away from storing too much of our own
metadata on the MeldBufferData class, trying to use built-in attributes
where we can. In this case, there's no real reason to try and track the
modified attribute, since GtkTextBuffer does a better job than we do.

This patch also highlights the crazy number of places we check this
stuff for e.g., setting save sensitivity, rather than binding like we
really should.