Fix widget margins on Mac with style sheets

Previously we have rejected this issue as you can work around it
by setting Qt::WA_LayoutUsesWidgetRect per widget. However there
is no reason to apply the negative mac style layout margins on any
custom style so I think we should completely bypass it unless the
native border is used.

Task-number: QTBUG-13050
Change-Id: I8923e07d868c51a13587993c9b2ce79c51beaeee
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Use QtPlatformHeaders to set platform specific functionality

This involves exposing a new function in the QPlatformNativeInterface
which gets a public function for QGuiApplication

Proof of concept is done through implementing _NET_WM_WINDOW_TYPE
setters for xcb

Change-Id: Ic9544e775fb71cc9b30273595ec41b1cdb1c9d64
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>