[webkit] editor.insertHtml not supported

[Garry Yao]

Currently we're currently rely on native document.insertHtml, but actually Webkit is not supporting this feature, check ​quirksmode test page.
It could be reproduced by the attached sample page.

[Garry Yao]

Changes contained within this patch:

1. Bring significant change to the way we insert html string, now it's using insertElement API under the hood which bring advantages of cross-browser dtd fixing;
2. Fixing a special case in selection.getRanges which will result in empty range when the document is initially focused;
3. Now the undosnaps happen for both 'insertHtml' AND 'insertElement' at API level.

[Garry Yao]

It's been proved that it's only a replication of #3703, and execCommand( 'inserthtml', false, data ) works exactly as it should.So this ticket is not valid.