- Remove margin-bottom for StIcon, which is not only ineffective, but
also created a bug in app folders.
- Remove ">" which is invalid for overview icons that are not app
folders.
- Apply spacing to the correct target "StBoxLayout", not the parent
.overview-icon.overview-icon-with-label.
Fixes https://gitlab.gnome.org/GNOME/gnome-shell/issues/2129