desktop: changes in appdata and 2.10 <release> now uncommented.

I was hoping others would review this <release> text before submitting
it to translators, but no such luck there. Anyway let's uncomment now
because translators need to get the text sooner than later if we want as
many localized version of the appdata text as possible at release.
Everyone if you want to review the English and the contents of this tag,
do it now!

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

@@ -72,7 +72,6 @@
   </provides>
 
   <releases>
-  <!--
     <release version="2.10.0" date="TODO">
       <description>
         <_p>
@@ -91,11 +90,11 @@
             On-canvas effect preview
           </_li>
           <_li>
-            Multi-threaded and hardware-accelerated rendering and
-            processing
+            Multi-threaded and hardware-accelerated rendering,
+            processing and painting
           </_li>
           <_li>
-            New and improved tools
+            Most tools improved, and several new tools
           </_li>
           <_li>
             Many image format support improved, in particular better PSD import
@@ -113,13 +112,15 @@
           <_li>
             Basic HiDPI support
           </_li>
+          <_li>
+            New themes for GIMP: Light, Gray, Dark and System
+          </_li>
           <_li>
             And much, much more…
           </_li>
         </ul>
       </description>
     </release>
-    -->
 
     <release version="2.10.0-rc1" date="2018-03-26" type="development">
       <description>