Standalone Editor: Improve Undo

[JiuqingSong]

Improve Undo Snapshot:

• Editor take an Undo Snapshots object instead of SnapshotsManager
• Editor API takeSnapshot now returns the new snapshot object
• Add an optional callback onChanged in Snapshots interface
• With all above, demo site now can easily get/set snapshots
• Demo site now supports copy snapshot with "metadata" format, and paste a metadata directly into each box (html, selection, ...)

This is a preparation step for new color manager.