ViewerFrame:
- Removed unused and empty implementation of ChangeListener interface.
- Split up actionPerformed method to make it more manageable: new handleZoomActions and handleMenuActions do part of the work.
- Zoom actions In, Original and Out now share a single loop to reduce code duplication.
- Created createLogoLabel method to reduce code duplication.
- Used "object instanceof SomeClass" instead of "object.getClass().getName().equals("some.package.SomeClass")".
- Changed some names.
- Put comments on a line of their own.
- Made some variables and parameters final.
- Added and improved Javadocs.
- Removed trailing spaces.

AboutFrame:
- Fixed some small Checkstyle issues.