data/schema: Add 'allow-override' gsetting

This allows to list apps that can override the currently
effective feedback level by setting the 'impotant' hint in
the event.

data/org.sigxcpu.feedbackd.gschema.xml

@@ -23,6 +23,15 @@
         feedbackd receives an event.
       </description>
     </key>
+
+    <key name="allow-important" type="as">
+      <default>[]</default>
+      <summary>List of apps that can override the feedback level</summary>
+      <description>
+        Apps listed here can override the configured feedack level by
+        setting the 'important' hint on the submitted event.
+      </description>
+    </key>
   </schema>
 
   <schema id="org.sigxcpu.feedbackd.application">