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/GcRideFile.cpp

@@ -173,7 +173,7 @@ GcFileReader::openRideFile(QFile &file, QStringList &errors, QList<RideFile*>*)
     if (present->name) \
         sample.setAttribute(#name, QString("%1").arg(point->name, 0, 'g', 11));
 bool
-GcFileReader::writeRideFile(MainWindow *,const RideFile *ride, QFile &file) const
+GcFileReader::writeRideFile(Context *,const RideFile *ride, QFile &file) const
 {
     QDomDocument doc("GoldenCheetah");
     QDomElement root = doc.createElement("ride");