MainWindow: Don't use get_active_view() to determine active session

MainWindow: Don't use get_active_view() to determine active session

While it would be neat if it worked, it unfortunately doesn't as
the currently focused item may not be related to any view - e.g.
when one of the tabs of the QTabWidget was clicked.
For this reason, we store the last session the user interacted
with and treat it as the currently focused session.