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/CMakeLists.txt

@@ -183,7 +183,7 @@ set(SOURCES
     HTML/WebSocket.cpp
     HighResolutionTime/Performance.cpp
     InProcessWebView.cpp
-    Layout/BlockBox.cpp
+        Layout/BlockContainer.cpp
     Layout/BlockFormattingContext.cpp
     Layout/Box.cpp
     Layout/BoxModelMetrics.cpp