fix crash on opening about:config in qt5 embedding

There are two aspects of this fix:
    1. not all qt5 apps are linked with qtwidgets thus it's better not to use
       QApplication in qt5 embedding (when QGuiApplication is used instead)
    2. qt5's QIcon can be fetched from current theme where icon names
       adhere to freedesktop standard. There is this bug
       https://bugzilla.mozilla.org/show_bug.cgi?id=662299
       to get rid of gtk names in favour of freedesktop names.
       This fix is a workaround for the bug.