weather: Follow GNOME Weather's location permissions

Our weather integration should follow GNOME Weather as closely as
possible, which means that we should respect its location permission
rather than using our own or none at all (which we can as a "system"
component and as geoclue's authorization agent).

https://bugzilla.gnome.org/show_bug.cgi?id=780252

js/misc/permissionStore.js

@@ -17,6 +17,13 @@ const PermissionStoreIface = '<node> \
       <arg name="app_permissions" type="a{sas}" direction="in"/> \
       <arg name="data" type="v" direction="in"/> \
     </method> \
+    <signal name="Changed"> \
+      <arg name="table" type="s" direction="out"/> \
+      <arg name="id" type="s" direction="out"/> \
+      <arg name="deleted" type="b" direction="out"/> \
+      <arg name="data" type="v" direction="out"/> \
+      <arg name="permissions" type="a{sas}" direction="out"/> \
+    </signal> \
   </interface> \
 </node>';