Structured SnipSpace

As snipsnap has evolved it now has some rudimentary structure in snipspace.

I would like to make some changes so that the structure is semantic and emergent.

Default inheritance

The behaviour in terms of parentage when a snip is created implicitly (i.e. when it is referenced before it is created) should be the same as that now provided in the 'new' button, by default, snips should be the children of the snip from which they were first referenced.

Multiple inheritance

is a heresy in Java - implement multiple interfaces instead - but need not be in snipspace, because inheritance is weak (a child snip inherits only a link to its parent, no other characteristics).

If you guys give me a clue (where to start looking, what classes I might need to modify) I'll give this a go.