Code review comment for lp://qastaging/~townsend/unity/lp1070715

This seems to work, and I don't see regressions (more important thing here! :)).

However, at this point you can remove the last_active_application_ var at all from the model.

Also, would be nice to have some test (even if this maybe somewhat already tested). I think there are some in test_switcher_model.cpp that should inspire you.