add possibility to check code with checkstyle

- checkstyle config file is located in catroid/ project
- projects use this config file (project relative - local)
- project checkstyle config (location of checkstyle config,
excludes,...) located in .checkstyle project file
- projects can be checked with rightclick-checkstyle-check Code...
Merge pull request #716 from Catrobat/checkstylePlayground

successfully tested