Bugfix/90 prevent graph cycles

[TheCrimsonKing92]

This PR resolves #90. A minimal change in compiler.js exposes a new detectCycles function, which is leveraged in editor.js for detection and prevention. Additionally, a new dialog is spawned when a cycle is prevented, and while I have endeavored to imitate the style of the repository, I made a stylistic choice to omit text centering in the new dialog box (though this can of course be added).

Comments, questions, and feedback are welcomed.

[maximecb]

Seems I merged this too fast. There is a bug:

editor.js:166 Uncaught ReferenceError: Model is not defined
    at Editor.detectCycles (editor.js:166:21)
    at UINode.portClick (editor.js:1123:24)
    at portDiv.onpointerup (editor.js:1146:23)

I think we should probably also move the detectCycles logic into the Model class.

Reverting for now.

[TheCrimsonKing92]

@maximecb I apologize, I see I missed an import during cleanup. I'll refactor and create a new PR with modified contents.

[maximecb]

No worries :)