VideoPlayer/LibVideo: Implement the UI functionality for seeking

With these changes, the seek bar can be used, but only to seek to the
start of the file. Seeking to anywhere else in the file will cause an
error in the demuxer.

The timestamp label that was previously invisible now has its text set
according to either the playback or seek slider's position.

Userland/Libraries/LibVideo/Containers/Demuxer.h

@@ -28,7 +28,7 @@ public:
         return sample.release_nonnull<VideoSample>();
     }
 
-    virtual DecoderErrorOr<void> seek_to_most_recent_keyframe(Track track, size_t timestamp) = 0;
+    virtual DecoderErrorOr<void> seek_to_most_recent_keyframe(Track track, Time timestamp) = 0;
 
     virtual DecoderErrorOr<Time> duration() = 0;