Fix 'cancel' and 'back' misbehavior.

- Check if the 'next' button should be enabled after 'cancel' or 'back'.
- Also, when the radio button is switched from existing to new provider,
check if we need to enable the 'check' button.

[Closes #4148]
Merge remote-tracking branch 'ivan/bug/4148_wizard-ui-providers-next' into develop