Event input:start fired twice for single click

If you single click on an `iro.ui.Box` (not on the handle) the `input:start` event is fired twice. Please see this pen:

https://codepen.io/RickMohr/pen/ExXXrOB

Open the console, and click on the box -- "input:start" is printed twice.
( It is correctly printed only once if you drag the box's handle or interact with the slider instead of the box.)

Start by reproducing the issue in the linked CodePen and tracing the event handling for an iro.ui.Box click, comparing it with the handle and slider paths. Done means a single click on the box emits input:start once while handle dragging and slider interaction retain their current behavior.

Written by the indexing model from the issue text.