MainWindow Refactor Part 5 of 5 - TABBED ATHLETES

The final part (and one of the reasons) for the mainwindow
refactoring -- we now support tabbed athletes rather than
having a new mainwindow for each athlete opened.

Context is saved/restored and there are new functions for
opening and closing tabs and windows of tabs.

The tabbar itself is fugly -- the next few days will spend
some time looking at making it prettier on Linux/Win and
more native on Mac (see MMTabBar).