Fixes#8533
Replace the usage of `Stacked` for error pages with programmatically
swapping the child of the `adw.Bin`.
I regret to say I don't know the root cause of this. I only know that
the usage of `Stacked` plus `Gtk.Paned` and the way we programmatically
change the paned position and stack child during initialization causes
major issues.
This change isn't without its warts, too, and you can see them heavily
commented in the diff.
(1) We have to workaround a GTK template double-free bug that is well known
to us: if you bind a template child that is also the direct child of the
template class, GTK does a double free on dispose. We workaround this by
removing our child in dispose. Valgrind verifies the fix.
(2) We have to workaround an issue where setting an `Adw.Bin` child
during a glarea realize causes some kind of critical GTK error that
results in a hard crash. We delay changing our bin child to an idle
tick.
ef822612d3