Allow user dictionary file to have custom location and to be a symlink

[otyu]

Using 2.19.2623.102 in Debian 9 (stretch). Currently, the user dictionary database is located on the file $HOME/.mozc/user_database.db. It is not possible to customize the location of this file, nor is it possible to replace it with a symlink (mozc seems to replace the link with an actual copy of the linked file).

Adding the possibility to place it on a custom location would help the user with more granular needs (e.g. sync'ing dictionaries issue 426). Allowing it to be a symlink would achieve the same results.

For example, the user could select the dictionary file on the Mozc Settings window, Dictionary tab, User diconary subtab.