Translation support for buttons and indicator

This patch to support the translation for the added buttons and indicator form this module, also i have changed the property currentStep inside FormStep class from protected to public as it doesn't make sense to define it as protected what if we need to access it from another form alter using the multistep_controller value inside the form alter.

i've spent 5 hours trying to override the indicators & buttons added from this module, but i guess you've code it in an "override proof", i guess you need to check the rebuildForm method maybe you need to use hide() instead of unset() as the passed form the the next form_alter hook won't have the full form object.

Thank You.