Fixes to Stage.nextStage docs.

Signed-off-by: Grant Skinner <info@gskinner.com>
ButtonHelper now sets mouseChildren to false. Added ButtonHelper.enabled g/s. Disabled pointer cursor when ButtonHelper is disabled.

Signed-off-by: Grant Skinner <info@gskinner.com>
Added ButtonHelper example.

Signed-off-by: Grant Skinner <info@gskinner.com>