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
The export functions in mainwindow are getting quite
cumbersome with multiple menu options.
This patch creates a single menu option "Export.." which
allows the user to select a supported format and a filename.
To support this the ridefile reader code needed to be adjusted
to allow registered readers to declare capability to write and
use a consistent (virtual) method to do so.
By modifying the base class for ride file reader we now allow
new readers to register both read and write capability.
