Fixed Checkstyle issues in the AboutFrame

AboutFrame:
- Placed many string concatenators (plus signs) on the next line to resolve these Checkstyle issues.
- Created the ICON_SIZE and OK_BUTTON_TEXT constants to fix some Checkstyle issues.
- Changed the createAboutScrollPane method into the addScrollPane method to make adding the scroll panes shorter.
- And some more changes to take care of the final Checkstyle issues.