LibWeb: Make DOM timers cancellable and stop leaking them

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.

Libraries/LibWeb/CMakeLists.txt

@@ -57,6 +57,7 @@ set(SOURCES
     DOM/ParentNode.cpp
     DOM/TagNames.cpp
     DOM/Text.cpp
+    DOM/Timer.cpp
     DOM/Window.cpp
     DOM/XMLHttpRequest.cpp
     DOMTreeModel.cpp