Improve behaviour when double-clicking on adder icon

_Originally reported as https://github.com/hypothesis/h/issues/3502 by @seanh._

### Problem you are trying to address

I recently got a new touchpad and the drivers apparently aren't tuned very well because I often double-click when I meant to single-click. Double-clicking when you only need to single-click is also something that I think many users often do either deliberately because they think they need to double-click on things, or accidentally because they lack excellent motor control, or simply out of habit (you don't have to double-click on links and buttons in browsers, but doing so does no harm).

Here's an example of one thing that can happen when double-clicking instead of single-clicking to create a new annotation:

1. The sidebar quickly opens and then closes again, I've made a separate issue for that here #189.
2. The adder popup disappears and reappears in a different location with different selected text. Let's discuss this part in this github issue'

### Your solution

I think the adder reappearing in a different location is because the browser responds to the second-click by creating a new selection. Since this click happened on our adder icon while it was still displayed, can we swallow the click to prevent it?

Start by tracing the adder icon's click handling and how the browser selection changes on a second click. Confirm the existing single-click behavior, then verify that a double-click on the displayed adder no longer moves the popup or changes the selected text; the issue does not name a file or test to run.

Written by the indexing model from the issue text.