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.
LibWeb now creates a WindowObject which inherits from GlobalObject.
Allocation of the global object is moved out of the Interpreter ctor
to allow for specialized construction.
The existing Window interfaces are moved to WindowObject with their
implementation code in the new Window class.
