This commit makes the following changes:

1. Improves the mutator code for use under the App Inventor blocks
rendering algorithm which walks the blocks tree starting from a given
node. Iterating over children resulted in an O(n^2) performance.
2. Prevents events from firing when the mutator is first opened, which
prevents superfluous rerendering operations before any changes have
been made by the user.
3. Defers resizing the mutator workspace on every block change to only
occur once at the end of the current JavaScript execution context via
setTimeout().

Fixes mit-cml/appinventor-sources#959