add additional checkstyle rules

### open for discussion

in this branch I enabled some more checkstyle rules which didn't raise any warnings or could be fixed easily!
have a look at the changes in catroid/checkstyle.xml - further infos for the added checks: http://checkstyle.sourceforge.net/availablechecks.html

some of these rules like GenericWhitespace were automatically resolved by our Eclipse settings when a file is saved and so did not need any code refactoring!
others like HideUtilityClassConstructor raised warnings and were resolved - see commit message for additional info!

also refactored some singletons to match our "convention" for singleton classes!

update # 1 - testrun: https://jenkins.catrob.at/job/Catroid-Multi-Job-Custom-Branch/783/

update # 2 - rebased; removed ArrayTrailingCommaCheck

update # 3 (29.11.) - newest testrun after rebase: https://jenkins.catrob.at/job/Catroid-Multi-Job-Custom-Branch/840/