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.

git-svn-id: https://develop.svn.wordpress.org/trunk@25748 602fd350-edb4-49c9-b593-d223f7449a82