model: Make Add word interface work

- Word is added but changes are not persistent. That is, it is lost
when application is launched next time. Will fix it later.
model: Make add word changes persistent in letter.properties file

Modify letter.properties file also if a word is added so that next
launch of application can reload it.

Remember that old letter.properties is in bin/ and that needs to
be replaced. Or project needs to be rebuilt for new file to take
affect.

Loading words from database is scheduled for next iteration now.