Add xdg-desktop-portal accent-rgb setting. (#191)

Author: mtwebster

schemas/org.x.apps.gschema.xml

@@ -78,5 +78,9 @@
       <default>false</default>
       <summary>Preferred contrast</summary>
     </key>
+    <key name="accent-rgb" type="s">
+      <default>''</default>
+      <summary>hex format RGB accent color</summary>
+    </key>
   </schema>
 </schemalist>