Replace deprecated code

[meisenzahl]

• ../src/Core/PackageKitBackend.vala:420.90-420.120: warning: `AppStream.ProvidedKind.MIMETYPE' is deprecated. Use MEDIATYPE
• ../src/Dialogs/StripeDialog.vala:192.43-192.57: warning: `Gtk.Dialog.get_action_area' has been deprecated since 3.12
• ../src/Views/AppListUpdateView.vala:323.17-323.38: warning: implicit .begin is deprecated
• ../src/Dialogs/RestartDialog.vala:81.31-81.45: warning: `Gtk.Dialog.get_action_area' has been deprecated since 3.12

[meisenzahl]

1. The action_area in RestartDialog gets only styled. Can we remove this code because Gtk.Dialog.get_action_area has been deprecated?
2. The action_area in StripeDialog gets styled and a Gtk.LinkButton is added. Can we replace the logic with Gtk.Dialog.add_button?

[danirabbit]

I think we should be able to do something like:

        var privacy_policy_link = new Gtk.LinkButton.with_label ("https://stripe.com/privacy", _("Privacy Policy")) {
            halign = Gtk.Align.START,
            hexpand = true
        };

        add_action_widget (privacy_policy_link, Gtk.ResponseType.NONE);

But to avoid having to manually mess with margins in dialogs, we need elementary/stylesheet#839