Added a JFace viewer filter for removing Coq output directories

Up until now, Kopitiam has been concealing output directories by marking
them as hidden resources. This isn't what the JDT does -- it installs a
Common Navigator filter, like this one. This is a much tidier approach, so
the next few commits will implement this behaviour for Kopitiam
Don't apply any resource flags to freshly-created bin/ folders

The upcoming Common Navigator filter will take care of hiding these for us
Added a Common Navigator filter definition for Coq output folders

This commit also adds this filter to the normal Project Explorer view; it
should also be contributed to the Java Package Explorer in future
Don't try to check closed projects for the Coq nature
Added the appropriate resource dialogs to the load path configuration page

Coq model objects have no mechanism for notifying clients of changes yet,
so these dialogs still don't do anything
Fixed a name resolution bug in the CoqBuilder's resolveRequire methods

Prior to this commit, qualified names were being treated as simple names
Added _CoqProject export support to the CoqProjectImpl class

The setLoadPathProviders method now writes the new load path to the
_CoqProject file
Moved the implementation of the Coq model into a new file

CoqModel was getting a bit cluttered; this change tidies things up a bit
Synchronise the project description with _CoqProject when CoqBuilder runs

I was considering putting this code in the CoqProjectImpl class, but this
approach seems safer -- in particular, it can also deal with manual changes
made by the user