LibHTML: Add LayoutNodeWithStyle class, make LayoutText style-less

Since LayoutText always inherits style, it shouldn't store any style of
its own. This patch adds a LayoutNodeWithStyle class to sit between
LayoutNode and everyone who wants to inherit from LayoutNode except
LayoutText :^)

Since LayoutText can never have children, we also know that the parent
of any LayoutNode is always going to be a LayoutNodeWithStyle.
So this patch makes LayoutNode::parent() return LayoutNodeWithStyle*.

Libraries/LibHTML/Layout/LayoutInline.cpp

@@ -3,7 +3,7 @@
 #include <LibHTML/Layout/LayoutInline.h>
 
 LayoutInline::LayoutInline(const Element& element, NonnullRefPtr<StyleProperties> style)
-    : LayoutNode(&element, move(style))
+    : LayoutNodeWithStyle(&element, move(style))
 {
     set_inline(true);
 }