Fix selection issues with pins overlapping unrelated links. Fix not being able to start new links when there is an existing link on a pin

[Auburn]

Includes changes from #124

In v0.4 of ImNodes it is possible to start a link on an pin that already has a link. As part of the input handling changes this is no longer possible, attempting to start a new link simply selects the existing link.

If there is a hovered pin a link should only be considered hovered if that link connects to the hovered pin. This avoids issues where if there is a link behind an unrelated hovered pin it would be considered hovered and cause odd interactions.

[Nelarius]

Thanks for the fix! Finally had some time to go over this. This is a useful fix, so I would happy to merge it as is. However, I'm curious to hear what you think.

BeginLinkInteraction is getting complex. I'm wondering if it might make more sense to handle the "pin is hovered" interactions before handling "link is hovered" in EndNodeEditor. I.e., first handle the (pin hovered | pin and link hovered) case, since we should never end up considering link selection when a pin is hovered.

Would this be something you'd have time to experiment with in this pr, or should we leave it for another time?

[Auburn]

I'm happy to leave it as is for now considering you are working on a large refactor. Can always revisit it in the future when it breaks 😛

[Nelarius]

Can always revisit it in the future when it breaks 😛

That sounds like a plan 👍 😄