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/Forward.h

@@ -10,14 +10,25 @@ namespace DevTools {
 
 class Actor;
 class Connection;
+class CSSPropertiesActor;
 class DeviceActor;
 class DevToolsDelegate;
 class DevToolsServer;
+class FrameActor;
+class HighlighterActor;
+class InspectorActor;
+class PageStyleActor;
 class PreferenceActor;
 class ProcessActor;
 class RootActor;
 class TabActor;
+class TargetConfigurationActor;
+class ThreadActor;
+class ThreadConfigurationActor;
+class WalkerActor;
+class WatcherActor;
 
+struct CSSProperty;
 struct ProcessDescription;
 struct TabDescription;