Return EGLNativeWindowType instead of window number in winId()

QEglFSWindow::winId() method was changed to return EGLNativeWindowType
EGL window handle instead of static window number as it recommends in
documentation.

QPlatformWindow documentation reads: "The platform
specific window handle can be retrieved by the winId function." and
also for winId() method itself: "Reimplement in subclasses to return
a handle to the native window".

Task-number: QTBUG-32564
Change-Id: I634c5b4d966b6aebde72518a2c39717d1b39af08
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>