LibWeb: Add basic <object> element support

This patch implements a simple <object> element with fallback content.
If the URL from the data attribute fails to load (including 404),
we render the DOM tree inside the <object> as fallback content.

This works by generating a different layout tree for the <object>
depending on the state and success of the data load. Since we cannot
currently do incremental layout tree updates, we have to force a
complete layout tree rebuild when the resource load finishes/fails.

Libraries/LibWeb/CMakeLists.txt

@@ -52,6 +52,7 @@ set(SOURCES
     DOM/HTMLIFrameElement.cpp
     DOM/HTMLImageElement.cpp
     DOM/HTMLInputElement.cpp
+    DOM/HTMLObjectElement.cpp
     DOM/HTMLLinkElement.cpp
     DOM/HTMLScriptElement.cpp
     DOM/HTMLStyleElement.cpp
@@ -95,6 +96,7 @@ set(SOURCES
     Layout/LineBoxFragment.cpp
     LayoutTreeModel.cpp
     Loader/FrameLoader.cpp
+    Loader/ImageLoader.cpp
     Loader/ImageResource.cpp
     Loader/Resource.cpp
     Loader/ResourceLoader.cpp