Add TinyMCE command to handle opening of the upload/insert popup, fix the...

Add TinyMCE command to handle opening of the upload/insert popup, fix the shortcut "Alt+Shift+M", fix the "image" button in DFW, fixes #17992, fixes #18697, see $17144

git-svn-id: https://core.svn.wordpress.org/trunk@18710 1a063a9b-81f0-0310-95a4-ce76da25c4cd