gtk_builder_connect_signals() is no longer necessary, because all the
setup that made it necessary to have this extra step is now done
automatically via the closure functions.
This is pretty unused and gets in the way of the next steps.
A potential side effect is that for templates the widget was passed as
the user data argument. If that turns out to be important, we have to
special case that situation.
This is useful to sketch out in GtkBuilder widgets in different states
all at once, so that we can check theming is right for them.
Add some initial UI files for primary-toolbar and inline-toolbar widgets.
