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/LayoutTableRow.cpp

@@ -0,0 +1,16 @@
+#include <LibHTML/DOM/Element.h>
+#include <LibHTML/Layout/LayoutTableRow.h>
+
+LayoutTableRow::LayoutTableRow(const Element& element, NonnullRefPtr<StyleProperties> style)
+    : LayoutBox(&element, move(style))
+{
+}
+
+LayoutTableRow::~LayoutTableRow()
+{
+}
+
+void LayoutTableRow::layout()
+{
+    LayoutBox::layout();
+}