Validate pom & rascal.mf using PathConfig logic

[DavyLandman]

Is your feature request related to a problem? Please describe.
Right now we have some validation of the rascal.mf file, but none for the pom.xml. In rascal there is quite some good messages coming from PathConfig calculation code, but we need to wire this correctly in the IDE, such that when an error is resolved, it's also cleared. Since users cannot delete diagnostics themselves.

Describe the solution you'd like

• report errors on open projects
• after changes to pom.xml / rascal.mf re-calculate and report new or removed errors
• strech goal: think about providing quick fixes, although that would require a lot more effort

Describe alternatives you've considered
Extend the rascal.mf validator. It has the benefit of a fast path for quick fixes, but it duplicates our logic.

Context
This is based on a discussion in #744

[jurgenvinju]

Great let's do this.

Quick-fixes, if wired through the Message API could work relatively easily. The reason is that each command for Rascal: is wired through one generic command: BaseWorkspaceService.RASCAL_META_COMMAND.

So I'm hoping that if we register a Diagnostic for pom.xml with this command name, the command will automatically lead back to our command handler.

[DavyLandman]

Right now our warnings in pom.xml do not have position information, so that would make this a lot harder to correctly report the place where it's wrong & quick fixes for them.

[jurgenvinju]

Good point!

[DavyLandman]

@PieterOlivier is working on that right now.