Slow performance in populateTargets findParent

[dermitzos]

We are using the community edition of jsPlumb to create graphs with a large amount of elements. These elements have multiple source & target selectors.

When starting a new connection because of the multiple targets and the amount of parents, it's really slow to find the element that the target is referring to. You can see an example of the delay here.

Could we add an optional field for the source/target elements where we could provide the parent ID in an attribute (something like data-jtk-parent-id) and in the populateTargets if this attribute is present, get the parent using document.getElementById(el.getAttribute('data-jtk-parent-id')) ?

From my tests, in our implementation this improves the performance dramatically, where the time to findParent at first is ~3000ms in certain cases and using this implementation the time reduces to 35ms.

I can provide an example/PR of the implementation too if needed