Add a grabber to the left border of the dialog

This MR was merged. Should the status of the issue be changed to "Fixed"?

While the MR was merged the bits that establish the css file. All the bits of the change that colorize the border have been removed / overriden. As a result this issue should stay active or maybe needs work. (as there is work to be done still)

Next Steps

• Decide what we want the "grabber" to look like
• decide what we want the border separating the edit / preview sections to look like.
• We've seen a number of really good examples. Perhaps the easiest next step is to list them here.

Postponing as the current plan is to prevent the preview panel from being resizable (aside from switching to full screen).