(Issue #8) FUTURE: Sprite Builder should be able to read .png files.

Save and Load now use .png files, instead of .ser files.
(Issue #14) NPE thrown when cancelling a Convert to PNG, when naming the file.

Null happened in the PNG converter. Checked the file name for being
empty, but cancelling returns the name as null.
(Issue #2) Sprite Builder can only make 25x25 sprites.

Size now defaults to 35x35. New Sprite lets the user adjust the size of
the image, and loading sets the size to the size of the .png loaded in.
(Issue #15) The icons in the menu are huge.

New icons, as well as numerous refactors to prepare for using EventBus.