* When a MCE view is selected, normal keystrokes (those without command/control) are blocked.
* Backspace/delete will successfully remove the view.
* Arrow handling will be added at a later point.
* When a MCE view is selected and content is pasted or injected (e.g. when an attachment is added), the content is added to the first text node after the view. If the view is at the end of the document or the next element is another view, a text node is injected into the DOM immediately after the selected view.
see #21390, #21812, #21813, #21815.
git-svn-id: https://develop.svn.wordpress.org/trunk@22210 602fd350-edb4-49c9-b593-d223f7449a82
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