non-modal option of Dialog doesn't work

[pixelzoom]

Factored out of #166 (create a Dialog base type). I tested the modal option of joist.Dialog, and it doesn't work as expected.

I set modal: true on line 108 of AboutDialog.js, ran a sim in requirejs mode, and opened the AboutDialog. It behaved exactly like a modal dialog.

As described in the requirements for #166, a non-modal dialog allows you to interact with the sim while the dialog is open. And the dialog must be closed via close button or some interaction with dialog’s component.

[pixelzoom]

My bad on this one. I was actually testing modal. Retested with non-modal and it seems to work OK.