Focus is immediately applied on open, may cause trouble!

[nickschot]

Currently we immediately apply the focus-trap even if an animation is still about to happen. This may cause problems if something like an ember-tether is rendered inside the modal as the modal might still be moving.

This could be solved by utilizing the checkCanFocusTrap hook: https://github.com/focus-trap/focus-trap#createoptions . We can pass it a promise that resolves once the animation is complete.

We might want similar behaviour when closing the modal

[pichfl]

This should be easier to implement once #672 is merged.

[marcin-wicha]

I've run into exactly this issue today while using a tooltip based on ember-tether.
It's rendered in the middle of the page as the modals slides in.

Both issues references as blockers for #672 have been merged, is there anything else blocking us?
I can submit a PR if you're short for time, I could just use a little more context.