Writing Flow: More reliable block clear via focus bubbling

[aduth]

This pull request seeks to improve reliability of the block click-to-clear behavior. The BlockSelectionClearer component in use works by checking whether the target of a click is the div rendered by the clearer. However, since there are often many block-level elements spanning full width between the selection clearer and where the click occurs (e.g. space adjacent the post title), clicking in these areas would not clear the block.

The changes proposed here instead assign the clearer as a (non-tabbable) focusable element which clears selection when the clearer div is the target of the focus. This way, even if the user clicks in a block-level element, if that element is not focusable, it will not be the target of the event.

Testing instructions:

1. Create a new post
2. Insert a block
3. Click to the left or right of the post title
4. Verify that the selected block is cleared

Edge cases to consider:

• Firefox / Safari focus events on button clicks
• Editing block settings in the sidebar

[aduth]

May as well drop the Pen I put together demonstrating the effect:

https://codepen.io/aduth/pen/RMPgOE

Previous behavior effectively required clicking the "Red" directly, such that intermediate non-interactive elements could prevent the deselection. Current behavior is reflected in the Pen, allowing clicking through the non-interactive elements.