Added support for creating synthetic CoqSteps to CoqEditorHandler

There are currently no circumstances in which synthetic CoqSteps will be
created, because they still interact badly with certain kinds of
backtracking
Handle synthetic steps properly when Coq rewinds too far
Treat standalone Coq comment blocks as synthetic steps
Don't allow Coq error markers to appear behind the completed marker

Kopitiam will now remove such markers when a CoqStepForwardRunner completes
Behave more like Coq when initialising the load path

Coq also adds the library's "user-contrib" directory to the load path, if
one is present
Bumped the version number to 0.3.5 and added a changelog entry