Generic property support for platform windows

Generic property support for platform windows

QPlatformNativeInterface can now contain generic window
properties in a QVariantMap, to facilitate communication
with the compositor and clients for certain platforms.
When window properties change, a signal is emitted from the
respective QPlatformNativeInterface instance. The properties
are intended to be read/writable from both client and server.

Change-Id: I7b42f7910d03c0d309add6c7dbb1c9b66ad22a3f
Reviewed-on: http://codereview.qt.nokia.com/3956
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>