LibVideo: Create Resuming handler for seek/buffer handlers to inherit

This allows the logic for keeping track of whether to resume to the
paused or the playing state when exiting these states. The new
StartingStateHandler also uses the class, since it can also be paused
and unpaused while waiting for samples.

The pause/play actions on the handlers inheriting from the resuming
handler will also now notify the owner that the state has changed so
that it can change icons, etc.

Userland/Libraries/LibVideo/PlaybackManager.h

@@ -114,6 +114,8 @@ public:
 
 private:
     class PlaybackStateHandler;
+    // Abstract class to allow resuming play/pause after the state is completed.
+    class ResumingStateHandler;
     class StartingStateHandler;
     class PlayingStateHandler;
     class PausedStateHandler;