LibWeb: Add inline-level iterator that enumerates items for line layout

This patch adds a new mechanism that allows InlineFormattingContext to
build line boxes incrementally instead of all-in-one go.

Incremental build will eventually allow much better support for CSS
floating objects.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -215,6 +215,7 @@ set(SOURCES
     Layout/ImageBox.cpp
     Layout/InitialContainingBlock.cpp
     Layout/InlineFormattingContext.cpp
+    Layout/InlineLevelIterator.cpp
     Layout/InlineNode.cpp
     Layout/Label.cpp
     Layout/LayoutPosition.cpp