Browser Op mode improvements

This is an attempt to improve the op mode in the browser for #792. I haven't quite implemented what we discussed - hopefully you'll agree that it's an improvement though. Rather than display a read-only op ui for a preview, this approach displays an editable one as before. When the Launch button is pressed, it then breaks off a copy into a floating OpDialogue, so all the OpDialogue backgrounding/progress reporting/error handling/retrying is used, but we maintain pretty much the same old interface otherwise.