insertElement on top of text selection

[Garry Yao]

Unlike "insertHtml", "insertElement" invoking that happens on a text selection in structure like table and list either leaves a malformed DOM or even an error.

The following tickets are associated:

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#6700

Insert element over selection breaks DOM

#6432

Error occurring when trying to insert a table over a highlighted list

[Garry Yao]

One idea is to throw the task to browser (by translating to editor::insertHtml) to gain the same result as the browser dependent "insertHtml", but then bugs are also inherited, on the other hand, it could be done with efforts that shapes range::extractContent to achieve the same goal.