Only allow focus events immediately after an interaction

Replace the autofocus prevention with suppressing _all_ focus events
that didn’t happen directly after an interaction (keypress or click)
with the page.

Strictly speaking, autofocus may only happen during page load, which
means that we should only prevent focus events during page load.
However, it is very difficult to reliably determine when the page load
ends. Moreover, a page may load very slowly. Then it is likely that the
user tries to focus something before the page has loaded fully.
Therefore focus events that aren’t reasonably close to a user
interaction (click or key press) are blurred (regardless of whether the
page is loaded or not -- but that isn’t so bad: if the user doesn’t like
autofocus, he doesn’t like any automatic focusing, right? This is
actually useful on devdocs.io). There is a slight risk that the user
presses a key just before an autofocus, causing it not to be blurred,
but that’s not likely. Lastly, the autofocus prevention is restricted to
`<input>` elements, since only such elements are commonly autofocused.
Many sites have buttons which inserts a `<textarea>` when clicked (which
might take up to a second) and then focuses the `<textarea>`. Such focus
events should _not_ be blurred.
Remove invalid/unnecessary option descriptions
Merge pull request #360 from lydell/last-interaction

Only allow focus events immediately after an interaction