replace last occurrences of getZone() with get()

Issue #129
fix Zone.setSize() to actually set width + height

Issue #129
fix TabZone

Fixes #129

TabZone was relying on the old behaviour of SMT.remove() and SMT.add(),
which for a while now only works for children of the root Zone, fix by
using super.remove() and super.add() instead
simplify SMT's add()/remove()

SMT's add()/remove() now just wraps SMT.sketch's add()/remove(), as
SMT's code for this was redundant and error prone.