Add interval sort function

.. added to the splitter handle context menu

.. also took IntervalItem.h and IntervalTreeView.h from MainWindow.h
   coz I was getting pissed with recompiling everything when changing it.
   Thats why so many other files have been changed in this commit.

Fixes #338.

src/AddIntervalDialog.cpp

@@ -18,6 +18,7 @@
 
 #include "AddIntervalDialog.h"
 #include "MainWindow.h"
+#include "IntervalItem.h"
 #include "RideFile.h"
 #include <QMap>
 #include <assert.h>