Use the correct attribute value when asking coqtop to perform a rewind

The old code was sending a curious hybrid interp-rewind command, which --
rather predictably -- didn't work
Removed most of the undo information from the CoqStep class

In order to keep the code vaguely comprehensible, Add LoadPath will not
receive special treatment after all
Implemented more of the StepBackJob class

Stepping backwards is now basically working, although stepping back past
the end of a proof -- and synthetic step support -- still needs some work
Perform the correct comparison when stepping backwards to the cursor
Clear the GoalViewer when the active editor's goals are null
Added support for stepping backwards into the middle of a proof

Excellent! All I need to do now is sort out synthetic step support
When an editor is closed, shut down its coqtop instance

Shutting down coqtop while a command is executing is quite likely to cause
problems, so, er, don't do that for the time being
Removed KCoqAction and its remaining subclasses

CoqStepUntilAction has been superseded by StepToCursorHandler, and
CoqRefreshAction is no longer necessary now that jobs automatically
retrieve up-to-date goal information
Don't update the underway position inside a StepBackJob

Handlers (and other users of StepBackJob) should now update the underway
position themselves
Automatically retract modified regions of Coq files

This is only a temporary solution -- in particular, DocumentState's role in
this process needs to be rethought
Moved the automatic retraction code into CoqEditor.scala
Removed the CoqCommands object

This can't possibly work with ideslave
Removed some vestigial references to the actor support library
Removed some deprecated and now-useless code

In particular, DocumentState no longer implements the CoqCallback trait
Always keep the goal viewer up-to-date, even if a job fails or is cancelled
Deleted the CoqResponse and CoqCallback traits
Added some early support for error markers to the ideslave-based jobs

Old markers aren't cleared properly yet, and the margin marker can often
appear in a misleading place, but this is still a good start
Aggressively clear problem markers whenever a Coq document is updated
Removed the CoqTop.writeToCoq method

The Java verification code was the only remaining user of this method, and
quite a lot of that will need to be rewritten anyway
Deleted most of the EclipseBoilerPlate object