Cocoa: support modal windows

Cocoa: support modal windows

Qt::WindowModal windows and dialogs are shown using [NSApp
beginSheet:modalForWindow:modalDelegate:didEndSelector:contextInfo:] as
long as they have a valid parent. Otherwise they are behave as
application modal.

Use the existing modal session support in the QCocoaEventDispatcher
(which was inherited from Qt 4) to support Qt::ApplicationModal
windows and dialogs. Some changes to this code are needed to ensure
proper behavior:

1. Window level modification is now done in
QCocoaWindow::recreateWindow() instead of in QCocoaEventDispatcher.
2. Make interrupt() use [NSApp abortModal] to stop a modal session
(previously we were freeing memory from under Cocoa's feet, causing
tools like valgrind and Instruments.app to complain)
3. Do not remove an item from a list and use a const reference to the
removed item immediately after (minor bug fix).

Also make sure that QCocoaEventDispatcher cleans up any modal sessions
and retained user input events on destruction (otherwise we leave
NSApplication in a weird state, which causes some autotest failures).

Change-Id: Iaeefa025400f324b5348b8c81a40384ef026efb4
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>