Add processing direction to 'active script' decorator

[Graham Dutton]

Currently, Proof Script Document decorators display if a script is active or locked. This enhancement suggests that the proof script decoration feature an arrow which is a view of the document Scripting State.

The decorator would now report the following:

 Document State         | Decoration
------------------------+-----------------------
Not locked, not active  | blank
Active script, no queue | pink
Active, processing      | pink with down arrow
Active, undoing         | pink with up arrow
Fully Processed         | blue

This would require another case be added to the decorator, querying ProofScriptDocument.ScriptingState, and two new decorator icons. These should be overlaid upon the existing Script State blue/pink icons.