LibDevTools: Implement enough of the protocol to inspect tabs

There is a lot needed all at once to actually inspect a tab's DOM tree.
It begins with requesting a "watcher" from a TabActor. It seems there
can be many types of watchers, but here we implement the "frame" watcher
only. The watcher creates an "inspector", which in turn creates a
"walker", which is the actor ultimately responsible for serializing and
inspecting the DOM tree.

In between all that, the DevTools client will send a handful of other
informational requests. If we do not reply to these, the client will not
move forward with the walker. For example, the CSSPropertiesActor will
be asked for a list of all known CSS properties.

Libraries/LibDevTools/DevToolsDelegate.h

@@ -6,7 +6,11 @@
 
 #pragma once
 
+#include <AK/Error.h>
+#include <AK/Function.h>
+#include <AK/JsonValue.h>
 #include <AK/Vector.h>
+#include <LibDevTools/Actors/CSSPropertiesActor.h>
 #include <LibDevTools/Actors/TabActor.h>
 #include <LibDevTools/Forward.h>
 
@@ -17,6 +21,10 @@ public:
     virtual ~DevToolsDelegate() = default;
 
     virtual Vector<TabDescription> tab_list() const { return {}; }
+    virtual Vector<CSSProperty> css_property_list() const { return {}; }
+
+    using OnTabInspectionComplete = Function<void(ErrorOr<JsonValue>)>;
+    virtual void inspect_tab(TabDescription const&, OnTabInspectionComplete) const { }
 };
 
 }