99b5e10acf
Commit c39ffa111 moved the signal handling from the controls- to the
background-group to enable scrolling on non-primary monitors.
However this broke scrolling on reactive overview elements as the
workspace switcher, as they're not descendants of the background.
To fix, move scroll-event handling to the overview group itself,
which is the common ancestor of all overview elements.
https://bugzilla.gnome.org/show_bug.cgi?id=768316