LibWeb keeps growing and the Web namespace is filling up fast.
Let's put DOM stuff into Web::DOM, just like we already started doing
with SVG stuff in Web::SVG.
This patch adds a Web::Timer object that represents a single timer
registration made with window.setTimeout() or window.setInterval().
All live timers are owned by the DOM Window object.
The timers can be stopped via clearTimeout() or clearInterval().
Note that those API's are actually interchangeable, but we have to
support both.
