Factor "insert" into it's own API

[dethe]

We currently insert new blocks from dragging, tapping, paste, and undo/redo. We have a lot of duplicated code (and bugs) because of this, and making changes to how it works (such as allowing wb-expressions to be dropped in with wb-steps by automatically wrapping them) has to be done in multiple places.

This is also some of our oldest and gnarliest code and long overdue for shining a light on. Ideally we can leverage the custom elements to have a more OO approach for this instead of the current mess of nested if/else.

Related to #1320