Static validation and problem markers for Flux files (closes #20)

Reports different problem types in editor and problems view:

- Errors for missing commands (not declared in flux.properties)
- Errors for successive commands with incompatible input or output
- Warnings for commands with missing @In annotation, infos for @Out
Support `tee` commands (stream-tee, object-tee) in Flux validator
Merge pull request #23 from fsteeg/validation

Static validation and problem markers for Flux files (closes #20)