Allow branching in control programs based on node types

[rensink]

This feature request depends on the implementation of FR #181 (node typing in control). It has two versions: a modest and an ambitious one, which are not necessarily mutually exclusive.

Modest request:
Provide a C-like 'switch' construct based on the type of a given node variable.

Ambitious request:
Implement dynamic dispatching of recipes based on a node type hierarchy.

An example on where this would be useful is the Java flow grammar.

Reported by: zambon