Browser+Ladybird+LibWebView: Handle trivial content APIs in LibWebView

The goal here is to reduce the amount of WebContent client APIs that are
duplicated across every ViewImplementation. Across our three browsers,
we currently:

    Ladybird - Mix some AK::Function callbacks and Qt signals to notify
    tabs of WebContent events.

    Browser - Use only AK::Function callbacks.

    headless-browser - Drop most events on the floor.

Instead, let's only use AK::Function callbacks across all three browsers
to propagate events to tabs. This allows us to invoke those callbacks
directly from LibWebView instead of all three browsers needing to define
a trivial `if (callback) callback();` override of a LibWebView virtual
function. For headless-browser, we can simply not set these callbacks.

As a first pass, this only converts WebContent events that are trivial
to this approach. That is, events that were simply passed onto the tab
or handled without much fuss.

Ladybird/Tab.h

@@ -37,6 +37,9 @@ public:
         HistoryNavigation,
     };
     void navigate(QString, LoadType = LoadType::Normal);
+    void back();
+    void forward();
+    void reload();
 
     void debug_request(DeprecatedString const& request, DeprecatedString const& argument);
 
@@ -54,11 +57,6 @@ public:
 public slots:
     void focus_location_editor();
     void location_edit_return_pressed();
-    void page_title_changed(QString);
-    void page_favicon_changed(QIcon);
-    void back();
-    void forward();
-    void reload();
 
 signals:
     void title_changed(int id, QString);