model: Letter: Add a simple iterator to walk through WordPictureSound

Add a simple iterator which can be used by view/controller to walk through
the words matching certain search pattern as typed by user.
Modify WordPictureSound to store letter word is associated with
model: Alphabet: Add a method to retrieve WordPictureSound object

Add a method to retrieve WordPictureSound Object associated with a
given word. This can be useful when user clicks on a word for edit.
model: Alphabet: Add a method to add a new word

This can be useful when using all words view user wants to add
a new word.
model: Alphabet: Add method to add new word

Last commit was not sufficient to add new word. This commit does
following.

- Copy sound file from source to resource dir.
- Copy image file from source to resource dir.
- Update letter.properties file to reflect new word
- Add the word to letter list of WordPictureSound objects.
model: Alphabet: Add a method to delete a word.

This removes the sound file and image file from appropriate directory.
It modifies letter.properties file to remove entries associated with
word.
Also removes word from the list maintained in letter object.
model: Alphabet: Add a simple interface to edit a word

I guess simplest is to delete old word and open a new one. Let us
see how does it go.
model: Alphabet: Fix edit word interface

It could happen that sound/image file name has not changed and it
could be deleted and never be found again. So first save the temp
copy of file and then add.
Merge pull request #23 from CS673/vivek-word-model

Vivek word model