Cocoa: Re-implement QCocoaScreen::topLevelAt()

The previous implementation used [NSApp orderedWindows]
which does not return NSPanel subclasses, which is
used by Qt dialogs and pops.

Use [NSWidow windowNumberAtPoint:belowWindowWithWindowNumber]
instead, which hit-tests on all window types. This
can potentially include windows from other processes
and non-Qt windows which needs to be filtered out.

Add EXPECT_FAIL to tst_MacGui::nonModalOrder. The
correct topLevelAt() implementation now exposes that
this test is failing.

Task-number: QTBUG-39322
Change-Id: I81afa3da964e08fe682802220d8fe81e9284205e
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>