desktop: prepare release of GIMP 2.10.8.

Please everyone, feel free to update the list of upcoming
changes/features, if you feel some particular change should be more
prominently exposed in this list (or at the opposite that some change is
not worth mentionning), or for rewording, etc.

(cherry picked from commit ba53a833b2)

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

@@ -72,6 +72,32 @@
   </provides>
 
   <releases>
+    <release version="2.10.8" date="2018-09-30">
+      <description>
+        <_p>
+          GIMP 2.10.8 is mostly a bug fix and optimization release. In
+          particular, it includes:
+        </_p>
+        <ul>
+          <_li>
+          Adaptative chunk size when rendering projections, improving
+          responsiveness dynamically
+          </_li>
+          <_li>
+          Detection of RawTherapee (version 5.5 and above) improved on
+          Windows
+          </_li>
+          <_li>
+          XCF compatibility information in the Save dialog more
+          understandable and discoverable
+          </_li>
+          <_li>
+          Various performance log tools added and log recording made
+          available in the Dashboard dock
+          </_li>
+        </ul>
+      </description>
+    </release>
     <release version="2.10.6" date="2018-08-19">
       <description>
         <_p>