TinyMCE: fix styling in the Charmap modal so the click targets are as large as...

TinyMCE: fix styling in the Charmap modal so the click targets are as large as the table cells. Remove the "Close" button and close the modal after inserting the special char. Fixes #27107.
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git-svn-id: https://core.svn.wordpress.org/trunk@27031 1a063a9b-81f0-0310-95a4-ce76da25c4cd