Change QObjectHelper API.

This is required to make easier to add new features to this class
without breaking the ABI and without making the API uglier.
QObject helper: better handling of enumerations

- automatically convert strings to Enum values during QVariant -> QObject
  serialization
- make possible to convert Enum values to strings (rather then int)
  during QObject -> QVariant conversion.
Drop deprecated Serializer::Serialize(QVariant v) API
Allow skipping NULL or invalid Variant.

Add flags in qobject2qvariant() allowing to decide if Null and Invalid
variants should be generated. This can be used to generate more compact
JSON structures.
Polish commit e54d615a2.

Use better names, polish documentation.
Release version 1.0.0