Introducing the PlatformPanel event type.

Introducing the PlatformPanel event type.

This event can be used by any platform plugin to implement
special application panels/overlayed menus.

Currently used by QNX only.

This replaces sending fake Qt::Key_Menu presses in the QNX
plugin. Qt::Key_Menu is already used when invoking context
menus with the keyboard.

Change-Id: I9c8f1743fd147a07c11883323800017376915ae1
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Nicolas Arnaud-Cormos <nicolas@kdab.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>