Ladybird: Allow creating new tabs with plain HTML

This will allow us to internally create a new tab with HTML, to be used
by the View Source command.

Ladybird/Qt/Tab.h

@@ -34,6 +34,8 @@ public:
     WebContentView& view() { return *m_view; }
 
     void navigate(QString);
+    void load_html(StringView, URL const&);
+
     void back();
     void forward();
     void reload();