DOC: Move detailed description to the top of class documentation page

Moves the detailed description in class documentation pages
to the top of the class. This is the most informative part of the page,
and this is what most of the people are looking for when they open the page.

Documentation/Doxygen/DoxygenLayout.xml

@@ -40,6 +40,7 @@
   <class>
     <briefdescription visible="yes"/>
     <includes visible="$SHOW_INCLUDE_FILES"/>
+    <detaileddescription title=""/>
     <inheritancegraph visible="$CLASS_GRAPH"/>
     <collaborationgraph visible="$COLLABORATION_GRAPH"/>
     <memberdecl>
@@ -76,7 +77,6 @@
       <related title="" subtitle=""/>
       <membergroups visible="yes"/>
     </memberdecl>
-    <detaileddescription title=""/>
     <memberdef>
       <inlineclasses title=""/>
       <typedefs title=""/>