desktop: start a <release> tag for 2.10.4.

The 2 major changes seem to be the much-awaited font loading in
background, and this new "Auto straighten" option on Measurement tool.

Please everyone, feel free to tweak the release description while it is
still commented out. Once we remove the comment tags, it will be
submitted for translation.

(cherry picked from commit dc8f44c6fa)

desktop/org.gimp.GIMP.appdata.xml.in.in

@@ -72,6 +72,27 @@
   </provides>
 
   <releases>
+    <!--
+    <release version="2.10.4" date="TODO">
+      <description>
+        <_p>
+          GIMP 2.10.4 includes a lot of bug fixes as well as various
+          optimizations. In particular fonts loading is not blocking
+          startup anymore. Instead any feature using fonts (such as the
+          Text tool) will now be blocked until fonts are fully loaded.
+          This change was highly requested on some platforms where font
+          loading was sometimes painfully slow.
+        </_p>
+        <_p>
+          The Measurement tool is augmented with a new feature, "Auto
+          straighten", allowing to rotate the active drawable (layer,
+          channel…) with the measurement line used as horizon.
+          As other transform tools, the feature will work on linked
+          drawables as well.
+        </_p>
+      </description>
+    </release>
+    -->
     <release version="2.10.2" date="2018-05-20">
       <description>
         <_p>