Ladybird: Do not assume the web view is embedded in a normal tab

The LadybirdWebView currently assumed it is viewed with a Tab instance.
This will not be true with the JavaScript console. This patch removes
this assumption by plumbing WebContent callbacks through a new protocol.
The Tab interface then implements this protocol.

Ladybird/AppKit/UI/Tab.h

@@ -6,18 +6,12 @@
 
 #pragma once
 
-#include <AK/URL.h>
-
 #import <System/Cocoa.h>
 
 @class LadybirdWebView;
 
 @interface Tab : NSWindow
 
-- (void)onLoadStart:(URL const&)url;
-- (void)onTitleChange:(NSString*)title;
-- (void)onFaviconChange:(NSImage*)favicon;
-
 @property (nonatomic, strong) LadybirdWebView* web_view;
 
 @end