Use a MonoReconciler instead of an IDocumentListener to perform retraction
Removed some now-fruitless attempts to get and set Coq shells

When it's running in ideslave mode, coqtop doesn't have a shell
Ported the Activator singleton to Scala
Removed the marker management code from JavaPosition

CoqTopContainer will probably perform some marker management in future, but
that'll have to be implemented using MarkerJobs
Added support for sending interrupts to coqtop on POSIX-like systems
Added a handler for sending interrupts to coqtop

Unlike the other handlers, this one is only active while Coq is busy
Made the CoqEditorHandler.getStepBackPair operate on CoqCommands

As a result of this change, Java editor handlers will also be able to use
this method
Added a stack of JavaSteps to JavaEditorState

This will be used when performing Java backtracking
Added some incomplete retraction handlers to the Java editor

There's no job to actually perform the retraction, so these handlers just
print their retraction information to the console for now
Made the InterruptCoqHandler class depend only upon CoqTopContainer

This change allows the class to work as the default handler for both Coq
editors and Java editors