OFINIFile: Add support for quoted keys / values

This is a much more logical way to handle leading and trailing
whitespaces and also seems to be used by a few other INI
implementations.

Additionally, this imports OFINICategory.h in OFINIFile.h so that
importing OFINIFile.h is enough - this should be less confusing - and
allows opening non-existant files, treating them like an empty file.