Make communication between progress-nav and interactions more transparent and maintainable

The new progress-nav (#4265,#4365) has a Submit button which is supposed to act as the Submit button for the card’s interaction if show_nav_submit_button=True (some interactions, such as MultipleChoiceInput, don’t need a Submit button so that flag is False).

1. Every interaction that uses the Submit button needs to have a callback attached for EVENT_PROGRESS_NAV_SUBMITTED, which is broadcast from the progress-nav when the user clicks on the Submit button (the interaction then performs its submitAnswer action). This is repetitive, we want that the interaction creator doesn’t have to manually define the callback for each interaction. This is also unnecessary because submitAnswer is originally defined on ConversationSkinDirective, so the progress nav should not need to go through the interaction directive to call it.
2. Similarly, every interaction needs to call setAnswerValidity whenever the answer changes in order to control whether or not the submit button is enabled or disabled.
3. The ItemSelectionInput is a special case that has show_nav_submit_button=False, but if there are >1 answer selections then the progress-nav still needs to show the Submit button. We want to remove this special case.

[seanlip]

Noted, thanks @bansalnitish!

@AllanYangZhou – would you mind taking a look at this?

[vibhor98]

No problem @bansalnitish