LibWeb+LibWebView+WebContent: Introduce a WebUI framework

When we build internal pages (e.g. about:settings), there is currently
quite a lot of boilerplate needed to communicate between the browser and
the page. This includes creating IDL for the page and the IPC for every
message sent between the processes.

These internal pages are also special in that they have privileged
access to and control over the browser process.

The framework introduced here serves to ease the setup of new internal
pages and to reduce the access that WebContent processes have to the
browser process. WebUI pages can send requests to the browser process
via a `ladybird.sendMessage` API. Responses from the browser are passed
through a WebUIMessage event. So, for example, an internal page may:

    ladybird.sendMessage("getDataFor", { id: 123 });

    document.addEventListener("WebUIMessage", event => {
        if (event.name === "gotData") {
            console.assert(event.data.id === 123);
        }
    });

To handle these messages, we set up a new IPC connection between the
browser and WebContent processes. This connection is torn down when
the user navigates away from the internal page.

Libraries/LibWebView/WebContentClient.h

@@ -47,6 +47,8 @@ public:
     void register_view(u64 page_id, ViewImplementation&);
     void unregister_view(u64 page_id);
 
+    void web_ui_disconnected(Badge<WebUI>);
+
     Function<void()> on_web_content_process_crash;
 
     pid_t pid() const { return m_process_handle.pid; }
@@ -142,6 +144,8 @@ private:
 
     ProcessHandle m_process_handle;
 
+    RefPtr<WebUI> m_web_ui;
+
     static HashTable<WebContentClient*> s_clients;
 };