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/web/application/themes')
- Correctly represent 'empty' values: 0, false, empty arrays are now
  represented as they are (and not as null or empty strings)
Call getDefaultValueConstantName just once
Method tweak
Merge branch 'master' of github.com:concrete5/concrete5-5.7.0