Esc to exit insert mode in embeds.

Currently, when an `embed` has the focus, `Esc` doesn't get Vimium out of insert mode.  This fixes that.

This is the right thing to do in the most common use cases, and for most users.  Unfortunately, it could cripple some flash-based sites and games.

Proposal:
- Introduce this into `post-1.46` on an experimental basis.

We (I, actually) need feedback as to whether this is the right thing to do.

Here's a (rather unfortunate) [test page](http://www.dumpert.nl/embed/6631539/8d9ff21c/).

See also #1211 and #1194.