use StringKey for Alg name, add IAlgorithm::setIndex

this patch does 3 things:

- makes Algorithm::index() const (as it should have been), set the initial value to 0 instead of 123 (!)
- adds a protected Algorithm::setIndex(unsigned int) method. this allows you to distinguish between clones. AlgResourcePool is the one who does the setting, and is declared as a friend to IAlgorithm
- turns Algorithm::m_name into a StringKey (from a string), so we can hash on the name of Algorithms more easily (a useful ability when dealing with Alg clones, and trying to treat them all the same way)

See merge request !125