build: Checkstyle now verifies line length of Java files (<=120 char)

[Oliver-Loeffler]

Added rule to check that line length in Java files not exceeds 120 chars.

Issue

Fixes #690

With the new checkstyle rule, one will now get warnings on overly long lines:

> mvn checkstyle:checkstyle
...
[WARN] PreferencesRecordDocument.java:134: Zeile ist 121 Zeichen lang (Obergrenze ist 120). [LineLength]
[WARN] PreferencesRecordDocument.java:156: Zeile ist 148 Zeichen lang (Obergrenze ist 120). [LineLength]
[WARN] PreferencesRecordGlobal.java:418: Zeile ist 125 Zeichen lang (Obergrenze ist 120). [LineLength]
[WARN] PreferencesRecordGlobal.java:423: Zeile ist 129 Zeichen lang (Obergrenze ist 120). [LineLength]
...

Progress

• Change must not contain extraneous whitespace
• License header year is updated, if required
• The PR name must follow the pre-defined format
• Verify the contributor has signed Gluon Individual Contributor License Agreement (CLA)