Clean up source code

[kovzol]

• Use Java standards: Capitalize file names, use standard package names: https://docs.oracle.com/javase/tutorial/java/package/namingpkgs.html, use camel case for variables and methods: https://www.javatpoint.com/camel-case-in-java.
• Add Java documentation for public methods.
• Remove unused classes and methods.
• Unify translation retrieval (remove obsoleted methods).
• Separate unfinished parts (e.g. the area method), including translations keys.