fast-forward implementation v1

[Yuyz0112]

related to #6

Since the currently 'play at any time offset' implementation is pretty simple,
there are many things we can do to optimize its performance.

In this patch, we do the following optimizations:

1. Ignore some of the events during fast forward.
   For example, when we are going to fast forward to 10 minutes later,
   we do not need to perform mouse movement events during this period.
2. Use a fragment element as the 'virtual parent node'.
   So newly added DOM nodes will be appended to this fragment node,
   and finally being appended into the document as a batch operation.
   These changes reduce a lot of time which was spent on reflow/repaint previously.
   I've seen a 10 times performance improvement within these approaches.

And there are still some things we can do better but not in this patch.

1. We can build a virtual DOM tree to store the mutations of DOM.
   This will minimize the number of DOM operations.
2. Another thing that may help UX is to make the fast forward process async and cancellable.
   This may make the drag and drop interactions in the player's UI looks smooth.