Remove ownership from constructors

[simpleigh]

The AbstractBlock constructor takes an initialRow: Row (sensible) and an ownership: BlockOwnership (possibly less so). Passing the ownership in the constructor is sometimes convenient, but it might be best to remove this:

• smaller constructors are good
• we can simplify methods by setting the parent in AbstractContainer rather than each createSix call (although this does still need the index anyway)
• this unlocks the possibility of creating a .subscribe() method on AbstractBlock offering more flexibility