Allow unconstrained variables to be edited

This pull requests removes the restriction of having constraints already set up when adding an edit variable. I added tests supporting this workflow, which I did find very useful I have already tried with Cassowary.js.

Note that in these tests I do not execute `begin_edit()`, `end_edit()`. I have found examples using Cassowary.js that do not neither, and this allows me to manage edit constraints manually, thus allowing interleaved edits.

This pull request addresses https://github.com/Nocte-/rhea/issues/34