Improve "modal-stepper" shared style

[NicoTuxx]

At the moment, multiple modals shared a common style by adding class "modal-stepper", css rules being present in /theme in parsec-cloud.
Multiple things to decide here:

• Is this usage specific to parsec-cloud only? if no, should be moved to megashark-lib
• If you don't know the class, you risk to remove it or forget it, without knowing what it is doing, it would be better to:
  • have it as a ms-modal parameter (if possible to use it)
  • OR make a ms-modal-stepper with the goal to provide the style only