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/Layout/LineBox.h

@@ -26,6 +26,7 @@ public:
     void trim_trailing_whitespace();
 
     bool is_empty_or_ends_in_whitespace() const;
+    bool is_empty() { return m_fragments.is_empty(); }
     bool ends_with_forced_line_break() const;
 
 private: