Fixup Athlete/DBAccess/MainWindow dependency

A bit of a mix of stuff, but basically the Athlete
class has sql stuff in it, that DBAccess used but
also referenced a session number in MainWindow.

Removed the interdependency and moved all code to do
with DB to DBAccess and out of MainWindow and Athlete.

At the same time needed to clean up a bit of memory
management and so introduced a MainWindow and Athlete
destructor.

We are now ready to do refactor part 4 to split MainWindow
into a new View class (that will be in each tab).