Reinstate term splitting on term update.

Originally introduced in [30241] and reverted in [30336], term splitting is
back and better than ever. Now with *more unit tests* and *improved treatment
of child terms*!

See #30335.
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When creating a post containing emoji, the post can be saved incorrectly if MySQL isn't using a character set that supports emoji.

This change prevents the save from occurring, so it can be handled correctly in the UI.

See #21212.

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If a saving a post fails, remove any invalid characters (such as emoji) from the primary text fields, then try to save it again.

See #21212.


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Flush cache for newly created term in `_split_shared_term()`.

The term itself does not have any cached values yet, but in some cases the new
term's taxonomy may need its cached hierarchy to be refreshed as a result of
the term splitting.

Props jorbin.
See #30335.
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WordPress 4.1-beta1
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TinyMCE: fix the cursor appearance while it is outside the editor body. Props miyauchi, fixes #30112.
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Editor: enable spellchecking of the post title field on the Edit Post screen. Props marsjaninzmarsa, fixes #30338.
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