Add "linked from" strip below header of linked pane

[martinmcclure]

To help users navigate, add a one-line strip to each linked pane indicating that the pane is linked, and some clue of what it's linked from.
At the left can be an icon representing the linked state, and to the right a short textual description of what it's linked from.
The icon can be a button to unlink the pane (implementing part of #43 in a different way).

Having some way to unlink panes will be a significant improvement.

[martinmcclure]

The linked-from strip has been added, but right now only shows useful text in inspector panes linked from evaluation sub-panes.