Added a change notification system to ICoqModel and CoqElementImpl

Nothing uses this system yet, but Coq editors will soon use it to restart
their Coq instances when projects' load paths change
Send Coq model change notifications when a project's load path has changed

The behaviour here still isn't ideal -- in particular, notifications should
also be sent when new subdirectories appear in the load path
Removed some unnecessary code from the Coq initialisation runners
Uninitialise a Coq editor when the load path of its project changes

This will force the load path to be sent over again when the next
CoqStepForwardRunner is executed
Fire Coq load path change events when directories are created or deleted