This patch makes Page weakable and allows page-less frames to exist.
Page is single-owner, and Frame is multiple-owner, so it's not sound
for Frame to assume its containing Page will stick around for its own
entire lifetime.
Fixes#3976.
This works everywhere right now, but it's obviously not going to stay
that way forever. :^)
Note that this does not advance the cursor correctly for whitespace
since the cursor is DOM-based and doesn't take whitespace collapsing
into account yet.
Each Web::Frame now has a cursor that sits at a DOM::Position. It will
blink and look like a nice regular text cursor.
It doesn't really do anything yet, but it will eventually.
