Force PARENT edge coords in the JSON editor.

[skieffer]

This ensures that Sprotty is able to draw the graph correctly, and is a part of the solution to eclipse/elk#901 .
It also resolves #8.

Note however that it won't do anything until elk-live is using a version of ELK that incorporates eclipse/elk#1071 .

It appears to be consistent with older/current versions of ELK, i.e. they appear to simply ignore the new property, and their behavior is unchanged.

[skieffer]

Question: Should it be made conditional on the selected ELK version?

I could move the added lines a bit lower, under here:

elk-live/client/src/json/editor.ts

Line 102 in 439ce96

let selectedVersion = versionSelect.options[versionSelect.selectedIndex].value;

and make them kick in only if the numerical value of selectedVersion is large enough.

The reason for doing so would be to avoid passing earlier versions of ELK a property,
org.eclipse.elk.json.edgeCoords, that they don't know about.

But in testing, it seems to me that they don't care. All earlier versions seem to work just fine, even though I'm passing them an unknown property.

[soerendomroes]

Question: Should it be made conditional on the selected ELK version?

I think this it is fine as it currently is. @Eddykasp do we have time for an ELK and elklive release this year?