inline editor initial implementation

This is initial implementation of inline editor.

Current functionality:
* Opens a functioning inline editor
* Clicking "Done" button saves changes back to main document

Known shortcomings:
* Only works on "margin" rule - hard-coded, for now. Need an API that returns list of supported shorthand property names.
* Edits shorthand rule. Need to convert to longhand in inline editor, and then back to shorthand when done.
* Undo (and some other edit functions) are not working