Cocoa: Remove duplicate enum.

Change-Id: I8a3b700710a1c47a31ddeed59ef73f1032ee7d09
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Cocoa: Simplify native view access.

Use NSWindow and NSView accessors on QCocoaWindow
instead of QGuiApplication and nativeResourceForWindow.

Change-Id: Iabee1e42dd119726d932f89c66a90810463d70db
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>