Removed the command, button and handler for manually triggering coqc

The Coq project builder is now the only way to compile proof scripts
Simplified the ProjectSourceLoadPath constructor

In order to add support for inter-project dependencies, the load path code
nedss to be cleaned up a bit. To this end, the ProjectBinarySourcePath
class will go away in the next few commits
Added another layer of indirection to the Coq project load path mechanism

Coq projects now contain ICoqLoadPathProviders; unlike ICoqLoadPaths, these
can represent more than one load path entry
Don't attempt to clean up folders that don't exist
Added support for inter-project load path references

This isn't ready for real use yet: the project configurations will also
need to be updated in order to invoke the builders in the right order
Tidied up the load path UI slightly

This is an incredibly minor fix; obviously a proper UI will be needed soon
Added keyboard shortcuts for the remaining "Set Printing" commands