add possibility to use our checkstyle within eclipse

I committed our checkstyle.xml config file (which is used on our Jenkins server) to the catroid project and all of our projects (catroid, catroidTest, ...) use this config (project relative checkstyle mode).
To check a project within Eclipse, rightclick on project name (multiselect possible), select checkstyle-check code with checkstyle.
Automatic check is disabled because it slows down the build process (project properties-checkstyle-activate will change the .checkstyle file) - but this behavior can be easily activated in the future!
Changes in project properties-checkstyle are not required, but will lead to a change of the corresponding .checkstyle file. Additionally the checkstyle config is protected against unintentional changes within project-properties->checkstyle->configure (Catroid Checkstyle local).
New checkstyle rules can be added either by adapting catroid/checkstyle.xml manually or by disabling the protection (project properties->checkstyle->Local check configurations->select->properties->disable protect checkstyle file - then you'll be able to configure Catroid Checkstyle local (but do not forget to "lock" the file afterwards).

Feel free to test it out - might be useful for developers to check their code before committing/pushing.