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/KmlRideFile.cpp

@@ -176,7 +176,7 @@ static StyleMapPtr CreateStyleMap(const char* id) {
 // Serialise the ride
 //
 bool
-KmlFileReader::writeRideFile(MainWindow *, const RideFile * ride, QFile &file) const
+KmlFileReader::writeRideFile(Context *, const RideFile * ride, QFile &file) const
 {
     // Create a new DOM document and setup styles et al
     kmldom::KmlFactory* kml_factory = kmldom::KmlFactory::GetFactory();