LibVideo+VideoPlayer: Convert playback event handler to callbacks

To pass events from LibVideo's PlaybackManager to interested parties, we
currently dispatch Core::Event objects that outside callers listen for.
Dispatching events in this manner rely on a Core::EventLoop. In order to
use PlaybackManager from LibWeb, change this mechanism to instead use a
set of callbacks to inform callers of events.

Userland/Applications/VideoPlayer/VideoPlayerWidget.h

@@ -51,8 +51,6 @@ private:
 
     void toggle_fullscreen();
 
-    void event(Core::Event&) override;
-
     virtual void drop_event(GUI::DropEvent&) override;
 
     DeprecatedString m_path;