model: Prepare for transition to full use of db

- Create some helper functions to de-clutter LoadResources()
- Add a function loadTheme() in theme manager so that a resource is
  loaded only in memory and not in db.
- Create a new function LoadDatabase which loads all words and
  themes in databse (if database is empty).
model: Load words from database at startup time
model: Stop modifying persistent letter.properties file

Now addition and deletion to database is working. So do not add/delete
words to/from persistent letter.proerpties file. Only modify databse.
Merge pull request #50 from CS673/vivek-model-db-changes

Vivek model db changes