LibWebView+UI: Add structures to hold context menu and action data

We currently duplicate a lot of code to handle application/context menus
and actions. The goal here is to hold the data for the menus and actions
in LibWebView. Each UI will then be able to generate menus from the data
on-the-fly.

The structures added here are meant to support generic and checkable
actions, action groups, submenus, etc.

Libraries/LibWebView/Forward.h

@@ -12,10 +12,12 @@
 
 namespace WebView {
 
+class Action;
 class Application;
 class Autocomplete;
 class CookieJar;
 class Database;
+class Menu;
 class OutOfProcessWebView;
 class ProcessManager;
 class Settings;