Overall clean-up on Headers and its subtypes

Motivation:

- There are still various inspector warnings to fix.
- ValueConverter.convert() methods need to end with the type name like
  other methods in Headers, such as setInt() and addInt(), for more
  consistency

Modifications:

- Fix all inspector warnings
- Rename ValueConverter.convert() to convert<type>()

Result:

- Cleaner code
- Consistency