Better representation of method parameters

- Let's try to retrieve the constant names (so that the output won't contain installation-dependent constant values, for instance `DIR_FILES_THEMES` instead of `'/git/concrete5-5.7.0/web/application/themes'`)
- Correctly represent *empty* values: `0`, `false`, empty `array`s are now represented as they are (and not as `null` or `''`)