gtkwindow: Fix "shadowed" checks for GTK grabs

gtkwindow: Fix "shadowed" checks for GTK grabs

We iterate here from the target widget up the toplevel checking
for the previous and new grab, there's however 2 bugs here:
- The check for is_shadowed was different to the check for was_shadowed
- The loop started with the assumption that the widgets did not hold
  a grab, just to change it if the grab widget was found. (or maybe
  it's the other way around? it's unclear with the differing checks
  for past/present state).

Make these checks consistent, and ensure we start with the right
assumption for the past/present grabbing state, and accounting that
new/old grab widgets may or may not be part of the pick stack.