DbHelper: Make method to get instance synchronized
The DbHelper classes should be a singleton, but then the
method to get the singleton instance should be synchronized, in
case several threads are trying to get hold of an instance.
DatabaseHelper: fix repoDirExist check againt db
DataManager: Only calc thumbnail size if file exists
Code cleanup: remove public, abstract qualifier from public interfaces
Code cleanup: remove unused imports
DataManager: Refactor by making a new private method
Make new private method, and use it from three places, to avoid code
duplication.
Make some public static methods private static, since they
are only used internally in the class.
Code cleanup: use public static, not static public
Merge pull request #71 from alfh/fix_code_review_changes

Code review changes