When using touch input to manipulate the selection there are no
mousedown/mouseup events that we can listen to and the
touchstart/touchend events are not triggered when manipulating the
OS-provided selection handles in the browser.

Instead listen for selectionchange events and show the adder in
response.

To avoid showing the adder every time the selection handle moves which
would be distracting, we instead buffer selectionchange events and only
show them after a pause.

When the user is using mouse input to make a selection, we ignore
selectionchange events so that the adder does not appear until the user
finishes making their selection.