Part of https://gitlab.gnome.org/GNOME/Initiatives/issues/1;
Sets up dual installation for flatpaks, allowing us to
build and run geary without affecting our system install.
This will allow testing of nightlies and CI artifacts as well
via the flatpak, so we can iterate on designs and share
bundles to test with.
This moves the individual in-app notification implementation on each of
the editor panes to the editor itself, reducing redundancy and allowing
the editor to issue notifications if needed (spoiler: it will).
This allows the panes themselves to manage their own headers, command
stacks, and so on. Break out the account list pane out so it's handled in
the same way as the others.
