main: Use session mode property for welcome dialog

We only want to show the welcome dialog in the user session, not on the login screen or during initial setup. We currently achieve that by explicitly checking for those mode names, but there are other modes like gnome-classic where the dialog is equally un- helpful. Support those cases by adding a session mode property that determines whether the welcome dialog should be enabled, so that modes can opt in or out of the feature themselves.

(Both the 'gdm' and 'initial-setup' modes are based on the 'restricted' mode, so this change does not affect them)

Fixes #4026 (closed)