Move code responsible for 'forgetting' a view outside of code responding to the
TerminalDisplay deletion.

This avoids a loop like this:

~MainWindow
=> ~QStackedWidget
=> ~TerminalDisplay
=> QObject::destroyed
=> ViewContainer::viewDestroyed
=> ViewContainer::removeViewWidget
   - internal cleanup
   - try to remove TerminalDisplay from QStackedWidget which is being deleted and
crash

Instead the code now does:

~MainWindow
=> ~QStackedWidget
=> ~TerminalDisplay
=> QObject::destroyed
=> ViewContainer::viewDestroyed
=> ViewContainer::forgetView (does the internal clean up)

And if one tries to explicitly remove a view, sequence is:

ViewContainer::removeView
=> ViewContainer::forgetView
=> ViewContainer::removeViewWidget

The patch also removes ViewManager::focusActiveView() because it causes a crash
when closing a TerminalDisplay as it tries to put the focus on the deleted
TerminalDisplay. I initially called it through a queued connection, but realized
it is actually not needed for focus to be passed to the correct view, so just
removed it.

BUG: 331724
REVIEW: 118839