Cocoa: Allow frameless NSWindow child QWindows

Showing, moving and resizing

Contrarily to what an NSWindow does to its NSViews, child NSWindows need
to be explicitly shown and hidden, and clipped if the parent NSWindow
changes geometry. Also, hiding an NSWindow will not hide its child
windows. This needed to be managed manually, adding 2 additional states
to QCocoaWindow to reflect whether a child window has been clipped out by
any ancestor geometry change, or hidden by any ancestor being hid. Also,
ordering out an NSWindow will remove it fromm its parent's child windows
array, making necessary to maintain a parallel list of child windows in
QCocoaWindow.

Stack order

Although child NSWindows can be ordered relatively to each other, they
need to be added again to be moved lower in the window stack. This also
means the windows above it need to be added on top.

Key (focus) status

One of the remaining issues, is to make sure the top level window keeps
the "key status" while still forwarding key events to the child window.

Keeping same event propagation

This use case is best illustrated with undocking QDockWidgets (if these
are child NSWindows). The main issue is to make sure the QDockArea will
get the mouse events right after undocking a dock widget. We used a similar
workaround as the "key status" problem, and manually forward the mouse
events to the dock area's QWindow.

Manual test, by Morten Johan Sørvig, included.

Task-number: QTBUG-33082
Task-number: QTBUG-22815
Change-Id: I50e34936fb82bff013e99f4bcb3bd0db0704c6ae
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>