Refactor MainWindow Part 2 of 5

Decoupled classes from MainWindow to reference Context
and Athlete (and introduced a couple of new headers).

We no longer pass around a MainWindow pointer to children
but pass a context instead.

There are still a few pieces left in MainWindow that need
to move to a better place;
    * Setting/clearing filter selection
    * Working with Intervals
    * Adding/Deleting Rides
    * Save on Exit

As mentioned previously there are lots of other parts to
this refactor left to do;
    * break MainWindow Gui elements into Toolbar and Views

    * migrate from RideItem and Ridelist to ActivityCollection
      and Activity classes that are not tied into gui elements.

    * introduce Application Context and AthleteCollection

src/IntervalTreeView.cpp

@@ -18,10 +18,10 @@
 
 #include "IntervalTreeView.h"
 #include "IntervalItem.h"
-#include "MainWindow.h"
+#include "Context.h"
 
 
-IntervalTreeView::IntervalTreeView(MainWindow *mainWindow) : mainWindow(mainWindow)
+IntervalTreeView::IntervalTreeView(Context *context) : context(context)
 {
     setDragDropMode(QAbstractItemView::InternalMove);
     setDragDropOverwriteMode(true);