system: Don't leave overview when activating most items

The current behavior wasn't designed, but was introduced in commit
84efaac5 to work around technical limitations when we were still
using external tools like gnome-screensaver or gnome-session-quit
to handle the actions. Those limitations are long gone, so it makes
sense to make the actions consistent with the corresponding keyboard
shortcuts: Leave the overview when launching an application, and leave
it alone otherwise.

https://bugzilla.gnome.org/show_bug.cgi?id=691900