Various minor cleanups.

- Suppress deprecation in DropDownListBoxSelectionChangedEventSubscriber.

- Format javadoc in EndNotify.

- Remove useless comments, improve method parameter names, remove
  redundant casts to float, remove commented-out code, improve
  formatting in Falloff.

- Invert if clause in ImageModeHelper to return immediately if
  imageModeProperty in getAreaProviderProperty is null rather than
  surrounding the entire method's logic in an 'if imageModeProperty !=
  null' clause. Also return null in the else clause since before the else
  would simply fall through and return null at the end of the method
  anyway. Do the same with getRenderStrategyProperty.

- Add @Override annotations where appropriate and format javadoc in
  Screen.
Add support for two imageMode's with ChangeImage.

The ChangeImage effect is not very, ahem, effective because it requires
you to use the same imageMode value for both active and inactive images.

This is a ridiculous constraint because one of the most common - and
useful - applications of ChangeImage is to modify a button's state using
a sprite sheet. For example, it can be combined with onHover or onClick,
etc, and the button sprite sheet can contain a "normal" (inactive)
sprite, a "hover" sprite, and a "clicked" sprite.

Unfortunately this setup requires the active imageMode and inactive
imageMode's to be separate. How to solve this problem?

It's elementary, my dear Watson.

Check for two new keys in the getImageModeProperty method of
ChangeImage, imageModeActive and imageModeInactive. If they are not
present we simply revert to the old behavior and check for the imageMode
key. This preserves backwards-compatibility because their use is
optional.

I've already tested it and it works fine.
Merge pull request #164 from 3xp0n3nt/minor-cleanup

Various minor cleanups.
Merge pull request #165 from 3xp0n3nt/double-imagemode-for-changeimage

Add support for two imageMode's with ChangeImage.