TinyMCE: improve hiding of the image/gallery popup buttons, fix selecting a...

TinyMCE: improve hiding of the image/gallery popup buttons, fix selecting a gallery in IE, don't leave empty paragraph after deleting an image with the popup button, fixes #22463, see #21390

git-svn-id: https://core.svn.wordpress.org/trunk@22857 1a063a9b-81f0-0310-95a4-ce76da25c4cd