Minor freeze break: gnome-shell: hot corner and prelight

[Owen Taylor <otaylor redhat com>]

Patch landed in gnome-shell on 3-16 prior to 2.91.92 that added
prelighting to items on the top bar to make it obvious that they
were clickable.

In this patch the case where the pointer is sitting on the 3x3
actor that is there to implement the hot corner for "Activities"
was missed (this actor is clickable, but not actually part of the
Activities button), this means that when you trigger the hot corner
you get: 

 start:             no-prelight 
 enter-button:      prelight
 enter-hot -corner: no-prelight
 in overview:       prelight (actually, "active" state)

This flashing is somewhat distracting and confusing, and it 
would be good to fix it:

https://bugzilla.gnome.org/show_bug.cgi?id=645751
 
Consider the patch very safe - biggest risk would be that in some
circumstance we've missed in testing and review the Activities
button might end prelighted when it shouldn't or vice-versa.

- Owen