Support close on mouseleave for tether dialog

[BrianSipple]

Hello,

Thanks for putting together a super-useful addon that tackles a really hard problem.

I recently had a tether-dialog modal that I wanted to have close on mouseleave events, so that it would behave as a hoverable “pop-out” modal — much like the one seen here in Google Keep.

After implementing this successfully in a local fork, I figured I'd PR it back and propose a few additions to the tether-dialog component. modal-dialog itself already has closeOnClick, and the implementation here follows a very similar pattern (though I localized the functionality to tether-dialog because that seems to be the only pattern where it would make sense).

I also added a dummy app example for the behavior and some corresponding acceptance tests. (As a bit of an aside, I couldn’t help but notice that some of the patterns in the dummy app’s application controller were a bit WET, so I felt somewhat awkward about adding to them. That said, I didn’t want to change the overall architecture as part of this PR, but would definitely be interested in subsequent contributions to do so 😄).

[lukemelia]

@BrianSipple Thanks for contributing this back. Seems like there are some failures on more recent versions of Ember.