We have a number of cases where properties should have their
default value overridden in a subclass, but haven't because thats
annoying to do. We also have properties where the absence of
an explicit value has context-dependent meaning.
Add a list of exceptions for these cases.
Catch an error that indicates the file looks like a template,
and then try again, this time with the template parsing API
of GtkBuilder. This is a little iffy, since we need to create
a 'fake' type and instance to pass in, but it works ok in
simple tests.
