Make mod() a template function
Add utilities for handling sockets

Added
- PiiSocket::fullName()
- PiiSocket::parentOperation()
- PiiAbstractOutputSocket::connectedInputNames()
- PiiAbstractOutputSocket::connectedInputCount()
- PiiProxySocket::connectedInputs()
- PiiProxySocket::isConnected()
Implement meta-data interface

Added PiiOperation::socketData() for querying socket-specific meta-data.

Implemented a mechanism for adding meta-properties to properties using
Q_CLASSINFO.