Add desktop login test, revise and rename check_desktop
    
    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).