Content Model Selection API step 3: Add new selection API

[JiuqingSong]

Add new selection related API:

• setSelection: set selection of a content model to the given content. If there is already other selections, clear them
• deleteSelection: Delete selected content, handle all kinds of cases
• mergeModel: consume setSelection and deleteSelection, to merge a given content model into an existing one and replace selected content
• insertContent: consume mergeModel API to insert given HTML content or DOM node or content model into existing content using content model
• insertImage: A public API to consume insertContent() to insert image into editor