bug-56: do the followings:

1. add download activity
2. add listadapter for language
3. change wordsstorage to support one db for a lang and category
combination.
4. add dialogutils
5. add errorutils
6. option dialog for partial downloaded db
I am curious about that we sure this function will be called after `DownloadDBActivity.onCreate()` right ? Because in line 40(`getRowCount(langCode)`) is assume `storage` is already initialize.