Start after Gala - #79

Copied from https://github.com/elementary/gala/blob/d408d394dbd9a6c6999df924b9c47599cea58c15/data/gala-x11.target

i looked at what GNOME does in they backend and it's seem to use graphical-session.target.

Okay so with the latest commit the session doesn’t crash like it did before but it takes a REALLY long time to start. So I think there’s probably still some issue here.

I wonder if we're waiting on Portals in Gala because we use access dialog: https://github.com/elementary/gala/blob/d408d394dbd9a6c6999df924b9c47599cea58c15/src/Dialogs.vala#L7

now the issue is the portal frontend trying to start the backend and falling systemctl --user status xdg-desktop-portal shows:

Failed to create app chooser proxy: Erro ao chamar StartServiceByName para org.freedesktop.impl.portal.desktop.pantheon: O tempo limite foi alcançado

There are two synchronous DBus connections (to gala, and the notifications daemon) being made in the constructors of the new Background portal code.

These should probably be watches instead of blocking the startup of the portal process, and it would allow the connections to be re-made if either gala or the notification daemon crashed or restarted.

Not sure if that's the cause of the long session load here, but we had a similar issue in the past, solved with this:
https://github.com/elementary/portals/pull/34/files

Tried here without success, the delay continues.

I believe that we should first switch to using systemd to start the session (It's bug elementary/session-settings#17)