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/AllPlotWindow.cpp

@@ -24,6 +24,7 @@
 #include "RideFile.h"
 #include "RideItem.h"
 #include "IntervalItem.h"
+#include "IntervalTreeView.h"
 #include "TimeUtils.h"
 #include "Settings.h"
 #include "Units.h" // for MILES_PER_KM