LibHTML: Parse descendant relations in CSS selectors

"div p" now generates a Selector with two components where the second
component is a type=TagName, value="p", relation=Descendant.

We still don't handle matching of these, but at least we parse them.

Libraries/LibHTML/CSS/Selector.h

@@ -18,6 +18,7 @@ public:
         enum class Relation {
             None,
             ImmediateChild,
+            Descendant,
         };
         Relation relation { Relation::None };
 

Libraries/LibHTML/Dump.cpp

@@ -155,6 +155,9 @@ void dump_rule(const StyleRule& rule)
             case Selector::Component::Relation::ImmediateChild:
                 relation_description = "{ImmediateChild}";
                 break;
+            case Selector::Component::Relation::Descendant:
+                relation_description = "{Descendant}";
+                break;
             }
             dbgprintf("    %s:%s %s\n", type_description, component.value.characters(), relation_description);
         }

Libraries/LibHTML/Parser/CSSParser.cpp

@@ -88,7 +88,7 @@ public:
     {
         consume_whitespace();
         Selector::Component::Type type;
-        Selector::Component::Relation relation = Selector::Component::Relation::None;
+        Selector::Component::Relation relation = Selector::Component::Relation::Descendant;
 
         if (peek() == '{')
             return {};
@@ -149,6 +149,10 @@ public:
                 break;
         }
 
+        if (components.is_empty())
+            return;
+        components.first().relation = Selector::Component::Relation::None;
+
         current_rule.selectors.append(Selector(move(components)));
     };