Fixes#1060.
In the process of adding this, I also decided to rewrite
mainwindow.get_window() for clarity (and also because flake8 was warning
about complexity).
Also adds some tests to the new-instance-target mechanism, in particular
a specific test for the issue in question.
With the addition of :window-only, it's no longer necessary to use
"Given I have a fresh instance" to clean up previous windows. This
greatly cuts down on the amount of process restarts that need to happen
to complete a full testing cycle.
I also made one of the tests more robust against order alterations, and
removed some unnecessary extra commands that were already implied by the
background on others.
