hterm: Add 'desktop-notification-bell' option.

If true, terminal bells emitted while the terminal does not have focus
trigger web notifications (http://www.w3.org/TR/notifications/).

I've tried to make the behavior relatively unobtrusive: web
notifications respect the audio bell squelch, and are all cleared when
the terminal gets focus or if the user clicks any notification.

BUG=chromium:208710
Change-Id: Ic114e7e71a2ad5d82b13211bf93010b65c5131f3
Reviewed-on: https://chromium-review.googlesource.com/203003
Reviewed-by: Robert Ginda <rginda@chromium.org>
Tested-by: Robert Ginda <rginda@chromium.org>
Added some support for using default copy handling.

Fixes paste in FF by focusing a content editable element before doing a paste.  Also adds a preference setting to use the default window controlled handling of copy.

Change-Id: Ia95823e801fb113006998014ef3a2f14a6512532
Reviewed-on: https://chromium-review.googlesource.com/203271
Reviewed-by: Robert Ginda <rginda@chromium.org>
Tested-by: Robert Ginda <rginda@chromium.org>