After dictionaries are installed, users can look up new words in the search
bar. Switch between different dictionaries by tapping a corresponding
dictionary title on the bottom.
The search result is in terms of the related dictionary. If the word is
unfound, try to use other dictionaries.
Looked-up words can be added to Vocabulary.
Users can divide the Vocabulary into groups. The words will be saved into
the specified group once the group is selected.