Many mostly automated code quality fixups

[StenAL]

IntelliJ and Maven were complaining about lots of warnings and offering automatic fixups for most of them. This PR integrates the following fixes:

• remove uses of deprecated boxed number constructors
• fix most warnings about unchecked collection usage
• remove redundant casts
• remove all unused imports
• use String.repeat() instead of for-loop for string building (available since Java 11)
• use enchanced for loop (i.e. for (Class item: collection)) where possible (available since Java 5)
• remove unnecessary unboxing of wrapped types, i.e. calls to .intValue() (available since Java 5)
• use diamond shorthand (<>) in generics where available (available since Java 7)
• replace C-style array declarations with Java style declarations
• remove unnecessary character escapes
• replace manual min/max calculations with Math.min/max calls
• remove redundant array initializations
• convert some Javadoc comments to normal multiline comments, remove other useless ones
• remove some unused imports
• remove redundant "throws" clauses
• remove redundant semicolons from empty catch blocks
• remove unnecessary toString calls
• use magic constants (e.g. Font.PLAIN) instead of integers where possible