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/LeftRightBalance.cpp

@@ -43,7 +43,7 @@ class LeftRightBalance : public RideMetric {
     void compute(const RideFile *ride, const Zones *, int,
                  const HrZones *, int,
                  const QHash<QString,RideMetric*> &,
-                 const MainWindow *) {
+                 const Context *) {
 
         total = count = 0;