ListView entries in single layouts (Fixed #14)

The homework data is now stored in "ArrayList<HashMap<String, String>>", and not in a single String array like before. This have some
advantages:
- better design in ListView (now a LinearLayout, ported by F-Droid application, thank you therefore)
- ID for deletion is now directly gotten from HashMap and do not have to be stored in SharedPreferences
 -> Import/Export now only imports/exports Homework.db
- now it's easy to solve the following problems:
 -> #12 Edit Homework
 -> #13 Mark homework as completed
 -> #15 Sort Homework