Don't rely on gala gsettings key (#861)

Author: lenemter

compositor/Background/BackgroundSource.vala

@@ -39,7 +39,7 @@ namespace GreeterCompositor {
             monitor_manager = display.get_context ().get_backend ().get_monitor_manager ();
             monitor_manager.monitors_changed.connect (monitors_changed);
 
-            gala_background_settings = new GLib.Settings ("io.elementary.desktop.background");
+            gala_background_settings = new GLib.Settings ("io.elementary.greeter-compositor.background");
             gala_background_settings.changed["dim-wallpaper-in-dark-style"].connect (() => changed ());
 
             gnome_background_settings = new GLib.Settings ("org.gnome.desktop.background");

data/compositor.gschema.xml

@@ -13,6 +13,14 @@
     </key>
   </schema>
 
+  <schema path="/io/elementary/greeter-compositor/background/" id="io.elementary.greeter-compositor.background">
+    <key type="b" name="dim-wallpaper-in-dark-style">
+      <default>false</default>
+      <summary>Dim wallpapers when dark style is active</summary>
+      <description></description>
+    </key>
+  </schema>
+
   <schema path="/io/elementary/greeter-compositor/keybindings/" id="io.elementary.greeter-compositor.keybindings">
     <key type="as" name="zoom-in">
       <default><![CDATA[['<Super>plus', '<Super>KP_Add']]]></default>