monitor-manager: Notify privacy screen changes on hotkey press

When privacy screen is changed and this happens on explicit user request
(that is not a setting change) we should notify about this via an OSD.

To perform this, we keep track of the reason that lead to a privacy
screen change, and when we record it we try to notify the user about.

When the hardware has not an explicit hotkey signal but we record a
change we must still fallback to this case.

Fixes: #2105
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1952>

src/backends/meta-settings.c

@@ -478,6 +478,18 @@ meta_settings_is_privacy_screen_enabled (MetaSettings *settings)
   return settings->privacy_screen;
 }
 
+void
+meta_settings_set_privacy_screen_enabled (MetaSettings *settings,
+                                          gboolean      enabled)
+{
+  if (settings->privacy_screen == enabled)
+    return;
+
+  settings->privacy_screen = enabled;
+  g_settings_set_boolean (settings->privacy_settings, "privacy-screen",
+                          enabled);
+}
+
 MetaSettings *
 meta_settings_new (MetaBackend *backend)
 {