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/GoogleMapControl.cpp

@@ -21,6 +21,7 @@
 
 #include "RideItem.h"
 #include "RideFile.h"
+#include "IntervalItem.h"
 #include "MainWindow.h"
 #include "Zones.h"
 #include "Settings.h"