add MutationObserver for Blacklight 8 Modal

[yetti]

Blacklight 8 uses an HTML Dialog element instead of a Bootstrap modal, so it won't fire the 'shown.bs.modal' events when opened. This adds a MutationObserver to check if the 'open' attribute is added to the HTML Dialog element, to determine if it's open.

[jcoyne]

Would it be simpler to have Blacklight itself emit an event that we can bind to here?

[yetti]

If we have Blacklight Modal emit its own events, doesn't that bring us part of the way back to the functionality that Bootstrap modals provided? I'm not across the reasons for moving to HTML Dialog in the first place, but it was definitely a loss of extensibility.

In any case, my reasoning for using a Mutation Observer was to not rely on a third party extension point again, but with what the HTML element provides natively (the "open" attribute).

[jrochkind]

If BL were to emit it's own event, then this func makes a future b-l-r rely on some future version of Blacklight (not work with current ones), as well as requiring PR's to both places.

If b-l-r just uses native functionality, there is less compatibility confusion, and a future b-l-r can work with current Blacklight versions.

On the other hand, of course the choice to use mutation observer means everyone that wants to observe needs to re-implement this behavior... but it's not that hard to implement is it? (Or is it?) I share yetti's assumption that BL's choice to use native modal was meant to keep things simple and tell clients to use native functionality; complex logic that changes from version to version of BL has been problematic in the past.