Don't lose state when loading tabs in the background

Don't lose state when loading tabs in the background

'locationChange' used to be detected in a rather complicated fashion. While this
worked mostly it had problems with tabs loading in the background.
'locationChange' wouldn't fire until the a background tab was _selected,_
causing any state, such as scrollable elements, collected up to that point to be
lost. It also meant that the blacklist patterns were applied to late, which
could cause unwanted autofocus prevention, for example.

Using the 'readystatechange' inside frame scripts seems to do exactly what we
want. This commit switches to that technique, which also simplifies the code a
lot, removing many slightly hacky parts.

See #605.