TinyMCE: add support for IE11.
- Add the latest changes from the 3.x branch upstream.
- Reduce the number of calls to tinyMCEPopup.restoreSelection() in our custom plugins.
- Remove the deprecated execCommand("mceBeginUndoLevel");.
See #25281.
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git-svn-id: http://core.svn.wordpress.org/trunk@25661 1a063a9b-81f0-0310-95a4-ce76da25c4cd