accounts-service: Add storage of OARS filter data

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://phabricator.endlessm.com/T23999

accounts-service/com.endlessm.ParentalControls.AppFilter.xml

@@ -31,5 +31,33 @@
       <annotation name="org.freedesktop.Accounts.DefaultValue"
         value="(false, [])"/>
     </property>
+
+    <!--
+      oars-filter:
+
+      A filter for which applications the user can see in app lists and install.
+      This is intended to be set by administrators and read by users, rather
+      than being editable by the user themselves.
+
+      It’s a two-tuple of the rating type, and a dictionary of rating sections
+      and values. The type gives the rating scheme in use — currently only
+      `oars-1.0` is supported.
+
+      Each dictionary entry is a mapping from an OARS section to the
+      most severe value for it which the user is allowed to see (inclusive). Any
+      app with a more severe value for any section must not be listed or
+      installable by the user.
+
+      An empty dictionary means that no OARS filtering is to be performed for
+      the user.
+
+      This setting is essentially equivalent to the `content_rating` dictionary
+      in AppStream data:
+      https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-content_rating.
+    -->
+    <property name="oars-filter" type="(sa{ss})" access="readwrite">
+      <annotation name="org.freedesktop.Accounts.DefaultValue"
+        value="('oars-1.0', @a{ss} {})"/>
+    </property>
   </interface>
-</node>
+</node>