JSSP, SVG and String editors should accept single-quoted strings

I made a modification to `$.utils.code.toSource` to use `$.utils.code.quote` instead of `JSON.stringify` to format strings (`.quote` prefers to use single quotes instead of double-quotes unless it would result in too many `\'`s), and in the process inadvertently broke the JSSP, SVG and String editors because they no longer comprehended the data to be edited.

Obviously I've reverted the change for now, and equally obviously it is a fool's errand to try to make the front ends fully general in what they can accept, but it would be nice for the editors to not be *quite* so dependent on the specifics of how the server chooses to represent the values to be edited.