use instancetype in sharedInstance-methods
fix when moving files if src and destination equals
always return valid path for preview image
fixes #221
Merge branch 'master' of gh:Catrobat/Catty into issue#221_2

* 'master' of gh:Catrobat/Catty:
  fix analysis issue (Value stored to 'url' is never read)
  animation improvement
  refactoring
  small improvement
  session improved with Image
  session try
Merge branch 'issue#221' of gh:Catrobat/Catty into issue#221_2

* 'issue#221' of gh:Catrobat/Catty:
  license header fix