enable more checkstyle rules and refactor code

- remove nested blocks
- change C style arrays to java style
- long constants should be defined with upper L instead of l.
    l looks like the digit 1
- add private constructor to utility classes!
    these classes contain only static methods/fields in their API,
    and it shouldn't be possible to instantiate them.
    private constructor is hiding implicit public default constructor!!!
- classes with only private constructors should be declared final
- each variable declaration should be in its own statement/own line
    according to several Code conventions/best practices
- refactored Singletons
Merge pull request #807 from Catrobat/checkstylePlayground

add additional checkstyle rules
Quickfix for issue #822 and update of test.
Merge pull request #824 from Catrobat/issue#822

Fix for issue #822 and update of test.
Merge branch 'master' into gradle

Conflicts:
	catroidSourceTest/src/org/catrobat/catroid/test/code/CheckForAssertionsTest.java
	catroidSourceTest/src/org/catrobat/catroid/test/license/LicenseTest.java