The approach of attaching sub-widgets to the web view widget was only
ever going to work in single-process mode, and that's not what we're
about anymore, so let's just get rid of WidgetBox so we don't have the
dead-end architecture hanging over us.
The next step here is to re-implement <input type=text> using LibWeb
primitives.
The WebContent process was redoing page layout every time you scrolled
the page. This was a huge CPU hog for no reason. Fix this by only doing
a relayout when the viewport is resized, not when it moves around.
Also stop exposing the DOM cursor as a mutable reference on Frame,
since event handling code was using that to mess with the text offset
directly. Setting the cursor now always goes through the Frame where
we can reset the blink cycle appropriately.
This makes cursor movement look a lot more natural. :^)
Image boxes want to know whether they are inside the visible viewport.
This is used to pause/resume animations, and to update the purgeable
memory volatility state.
Previously we would traverse the entire layout tree on every resize,
calling a helper on each ImageBox. Make those boxes register with the
frame they are interested in instead, saving us all that traversal.
This also makes it easier for other parts of the code to learn about
viewport changes in the future. :^)
