Add node placement options

Instead of adding nodes to the tail of their target group allow four
different placement options (head of group, tail of group, before node,
after node).
Remove Methcla_Resource from plugin API

Remove argument from Methcla_SynthDef::construct and rename resource
reference counting functions.

Conflicts:
	engine/ChangeLog.md
Remove unused enum
Refactor API command error reporting
Add support for node done actions

Add the possibility of performing an action when a node is done
processing.
Make doneFlags a property of Synth
Update ChangeLog