Allow wizard buttons to change enabled state depending on the user actions in the wizard form

We need to implement a mechanism to be able to disable a wizard button (next, finish) depending on the state of the wizard step form: e.g. if the user didn't make a mandatory selection / fill in a mandatory field, the "Next" button should not be enabled, etc. Currently, the buttons are enabled and disabled (visible invisible) per wizard step, but they cannot change state during one step.

The UI of this should be implemented according to XWIKI-4242.