TinyMCE: add a custom jQuery event 'tinymce-editor-init' triggered on...

TinyMCE: add a custom jQuery event 'tinymce-editor-init' triggered on initialization of every editor instance. This makes it a lot more convenient to hook into the instance compared to the init.setup callback. See #24067, see #26872.
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git-svn-id: https://core.svn.wordpress.org/trunk@26860 1a063a9b-81f0-0310-95a4-ce76da25c4cd