Cocoa: fix window focus lost when opening combobox

Cocoa: fix window focus lost when opening combobox

In cocoa, saying that a panel accepts key events, will make it receive
key events, but also show it as the active window on screen. The former
we dont really have to care about, since Qt will take care of forwarding
events to the popup for us anyway, even when they target another window.
So the only reason to actually let a panel become key window, is when
we want it to become active. And for popups, we only want this to happend
for Tool windows.

Change-Id: Ic4e5058307c514cbe30174d2a2d4ca0f41c8f71f
QTBUG: 26598
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>