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/TableRowGroupBox.cpp

@@ -12,7 +12,7 @@
 namespace Web::Layout {
 
 TableRowGroupBox::TableRowGroupBox(DOM::Document& document, DOM::Element& element, NonnullRefPtr<CSS::StyleProperties> style)
-    : Layout::BlockBox(document, &element, move(style))
+    : Layout::BlockContainer(document, &element, move(style))
 {
 }