[Doc] Improve topics within App

- The location of the documentation is improved (not in the cpp file
  anymore but in core-app.dox).  This prevents cluttering source with
  high-level overviews typical of topic documentation.
- The formatting has been made consistent.

src/App/Expression.cpp

@@ -52,11 +52,6 @@
 #include "ExpressionParser.h"
 
 
-/** \defgroup Expression Expressions framework
-    \ingroup APP
-    \brief The expression system allows users to write expressions and formulas that produce values
-*/
-
 using namespace Base;
 using namespace App;