This patch adds "submit" inputs and default (text box) inputs, as well
as form elements that can be submitted.
Layout of input elements is implemented via a new LayoutWidget class
that allows you to put an arbitrary GWidget in the layout tree.
At the moment, the DOM node sets the initial size of the LayoutWidget,
and then the positioning is done by the normal layout algorithm.
We also now support submitting a <form method="GET">, which does a full
replacing load with a URL based on the form's action + a query string
built from the name/value of input elements within the submitted form.
This is pretty neat! :^)
The <br> element will produce a special LayoutBreak node in the layout
tree, which forces a break in the line layout whenever encountered.
This patch also makes LayoutBlock use the current line-height as the
minimum effective height for each line box. This ensures that having
multiple <br> elements in a row doesn't create 0-height line boxes.
