Sandbox designer changes, part 1:

* Remove up/down/delete buttons from designer
* Move "Sandbox" component into its own file
* Change event preview handling in designer to trap more mouse events, so checkboxes don't toggle when you try to select them

Bonus bug fix: Selecting a kind in the designer was causing an update to be sent to the editor. This would cause the wrong kind to get its components overwritten by the newly-selected kind's components.