InsertMode should not handle key events if document.body is editable

For example, the host JavaScript may create an "about:blank" iframe with a
`content-editable` `body`, and then:
* the parent frame may handle `Escape` key events by itself
* but now, Vimium always grabs `Escape` events
  * and tries to exit `InsertMode`
  * although `document.body` is still current `activeElement`
    after `body.blur()`
* as a result, neither the parent can receive and handle wanted keyevents,
  nor Vimium will succeed in returing back to NormalMode

Steps to reproduce:

- Press and hold `j`.
- Tab `k`.
- Release `j`.

The tap on `k` is uninstalling the `cancelEventListener` installed by
`j`, and because we advance `@time`, the `j` stops scrolling and
removes the `cancelEventListener` installed for `k`.  So we end up with
no `cancelEventListener` installed.

The fix is to make the `cancelEventListener` specific to the scroller
instance.

The more fundamental problem here is that we're mixing dynamic and
static state.  A better approach would be to have `CoreScroller` as a
class, with a new instance created for each scroll instance.

Example page: http://codeforces.com/contest/752/problem/B.

There, `element.tagName` is an element with `name` `tagName` (not a string).  Here, we guard against that case.

Fixes #2305.

Try the following....

- load page
- `/`
- type some junk which matches nothing
- `Escape`

We expect to leave find mode.  Instead, we have to hit escape again to
get out of find mode.

Here, `focusNode.focus()` is not a function.

New command option to enter normal mode