Users: Drop deprecated GtkDialog/GtkMessageDialogs while improving error handling UIs

• This ports the "Add User" dialog from GtkDialog to AdwWindow.
• This ports some of the GtkMessageDialogs (the ones with actions) to AdwMessageDialog.
• This ports the message dialogs with errors (the ones with no actions) to toasts (AdwToast).

While porting away from GtkDialogs/GtkMessageDialogs in the Users panel I ended up doing a few collateral fixes.

• Dropped the empty state page "no-users-box" because that was only shown when AccountsService is not running, meaning that this should be more of an error page rather than a no-users-page. Specially because something is really wrong if you are in the Users panel without an user existing (shouldn't happen).

Before

Notice how we would advise the user to unlock the panel, when actually there was nothing that could be done in the panel given that there's no account service available.

After

• Added a loading page with a spinner to be shown before the panel gets populated. Otherwise you see the widget allocations moving things back-and-forth. This could be even more sensible with remote/enterprise accounts being loaded.