Fixes #63, refs #81

Next Composer Eclipse Plugin Updates

Stuff to do:

- [x] `isValidJson()` method on `IComposerPackage`
- [x] Make `BuildPathManager` to check wether the `IComposerPackage` `isValidJson()`
- [x] Move toggle Action and respective ToolbarContributiionItems from `ComposerFormEditor` to `DependencyGraphPage`
- [x] when there is a json invalid message, deactivate toolbar contributions on `DependencyGraphPage` to show the error message instead, also deactivate the graph
- [x] Check for correct dirty-state-changes
- [x] Check for potential lock-ups, when composer.json get's out-of-sync
- [x] Fix Test-Suite
- [ ] Write documentation on how to set up eclipse for running tests -> https://github.com/pulse00/Composer-Eclipse-Plugin/wiki/setting-up-your-development-environment