LibHTML: Add stub classes for basic table layout

This class introduces LayoutTable, LayoutTableRow and LayoutTableCell.
These are produced by "display" values table, table-row and table-cell
respectively.

Note that there's no layout happening yet, I'm just adding the classes.

Libraries/LibHTML/Layout/LayoutTableCell.cpp

@@ -0,0 +1,11 @@
+#include <LibHTML/DOM/Element.h>
+#include <LibHTML/Layout/LayoutTableCell.h>
+
+LayoutTableCell::LayoutTableCell(const Element& element, NonnullRefPtr<StyleProperties> style)
+    : LayoutBlock(&element, move(style))
+{
+}
+
+LayoutTableCell::~LayoutTableCell()
+{
+}