Added extra Checkstyle check
- added an extra throws related indentation Checkstyle check, which was depending on having Checkstyle 5.7 support.
- pom.xml: use maven-checkstyle-plugin version 2.12.1 (instead of version 2.11), which supports Checkstyle 5.7.
- BaseGuiExample: fixed a simple Checkstyle issue (duplicate string).