This adds a new test that implementsQA:Testcase_desktop_login
on both GNOME and KDE.
While working on this, we realized that the "desktop_clean"
needles were really "app menu" needles, and for KDE, this was
a duplication with the new "system menu" needles, because on KDE
the app menu and the system menu are the same. So I (Adam)
started to de-duplicate that, but also realized that "app menu
button" is a much more accurate name for these needles, so I was
renaming the old desktop_clean needles to app_menu_button. That
led me to the realization that "check_desktop_clean" is itself a
dumb name, because we don't (at least, any more, way back in the
mists of time we may have done) do anything to check that the
desktop is "clean" - we're really just asserting that we're at a
desktop *at all*. While thinking *that* through, I *also* realized
that the whole "open the overview and look for the app grid icon"
workaround it did is no longer necessary, because GNOME doesn't
use a translucent top bar any more. That went away in GNOME 3.32,
which is in Fedora 30, our oldest supported release.
So I threw that away, renamed the function "check_desktop",
cleaned up all the needle naming and tagging, and also added an
app menu needle for GNOME in Japanese because we were missing
one (the Japanese tests have been using the "app grid icon"
workaround the whole time).
Since the KDE menus have transparency set, any time the wallpaper
changes the menus will look different, and the app tests may
fail. This sets the desktop wallpaper to black at the start of
the test suite to avoid this problem.
