LibWeb: Move line boxes from Layout::Box to BlockContainer

Per the spec, only a BlockContainer" can have line boxes, so let's not
clutter up every Layout::Box with line boxes.

This also allows us to establish an invariant that BFC and IFC always
operate on a Layout::BlockContainer.

Note that if BlockContainer has all block-level children, its line boxes
are not used for anything. They are only used in the all inline-level
children scenario.

Userland/Libraries/LibWeb/Layout/InlineFormattingContext.cpp

@@ -15,7 +15,7 @@
 
 namespace Web::Layout {
 
-InlineFormattingContext::InlineFormattingContext(Box& containing_block, FormattingContext* parent)
+InlineFormattingContext::InlineFormattingContext(BlockContainer& containing_block, FormattingContext* parent)
     : FormattingContext(containing_block, parent)
 {
 }