GtkDialog is being deprecated. Instead of using it,
we want to subclass AdwWindow. As part of this change
the content and headerbar are put together in one box.
These are still hardcoded instead of measuring the available space,
but it's better than nothing in the meantime.
Decent WMs size the window correctly if width/height exceed the screen.
Also, allow the Notes field to expand vertically, to fill the space.
Fixes https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/602
Using dialog responses to communicate intent to GcalWindow was always
a hack, and as the code is cleaned, it's getting increasingly annoying.
Replace this with a proper 'remove-event' signal ('delete-event' would
conflict with the equally named GTK signal), remove the custom response
types, and cleanup the code.
Christopher Davis