Support (multiple) inheritance by scope completion
The code simply checks for inherited classes, strips unneeded stuff
like templates, etc. from inherited classes, detects multiple inheritance
by splitting the string using "," and calling tm_workspace_get_parents()
recursively on every parent class and collecting the returned tags.
The code limits the recursion depth to 10 to avoid possible inheritance
cycles and infinite recursion.
Also remove #if 0'd old code from TM implementing inheritance that we
kept for reference as we now have a better implementation.