LibWeb: Rename Layout::BlockBox => BlockContainer

There's a subtle difference here. A "block box" in the spec is a
block-level box, while a "block container" is a box whose children are
either all inline-level boxes in an IFC, or all block-level boxes
participating in a BFC.

Notably, an "inline-block" box is a "block container" but not a "block
box" since it is itself inline-level.

Userland/Libraries/LibWeb/Layout/LineBox.h

@@ -28,7 +28,7 @@ public:
     bool is_empty_or_ends_in_whitespace() const;
 
 private:
-    friend class BlockBox;
+    friend class BlockContainer;
     friend class InlineFormattingContext;
     NonnullOwnPtrVector<LineBoxFragment> m_fragments;
     float m_width { 0 };