Slight redesign, adding toggling optimization.

Blocks for trap:read: family are now `:model :html` instead of `:brush :model:`.

`trap:toggle:` added to show/hide only when changed to notNil/isNil.
This is an optimization to prevent frequent redraws.

Default-specifiable version of some methods added (old methods just
forward with a sane default):
 - `trapShow:default:`
 - `trap:toggle:ifNotPresent:`