Use setNSToolbar from the cocoa platform plugin

The platform plugin will attach the toolbar to the
NSWindow when the NSWindow is created.

Change-Id: I550516a240df1f8ae94da2a225d9f63c76fffc6b
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>