Prevent window "flashing" when closing the last session

There are two scenarios when closing a window:
A) clicking the close button on the title bar (or Ctrl+Shift+Q):
~MainWindow()
~ViewManager()
~TabbedViewContainer()
~TerminalDisplay()
~Session()

B) closing the last session/tab in a window:
SessionController::sessionFinished()
~Session()
~TerminalDisplay()
~TabbedViewContainer()
~MainWindow()
~ViewManager()

the issue with the second case is that the TerminalDisplay is torn down
first, which exposes the TabbedViewContainer widget, the latter has the same
Qt::Window colour as the system colour scheme window background colour, if
you're using a dark terminal colour scheme and a light-coloured system colour
scheme, you could see some "flashing" when you close the last session with
e.g. Ctrl+D.

To fix this, in sessionFinished() check if TabbedViewContainer::count() is
1 (i.e. closing last tab/session), and emit the empty() signal in that case,
which is connected to MainwWindow::close(), then the order of tear down
becomes:
SessionController::sessionFinished()
~Session()
~MainWindow()
~ViewManager()
~TabbedViewContainer()
~TerminalDisplay()

BUG: 432077
FIXED-IN: 21.12
(cherry picked from commit bbec7225)