Improve the image drag-resize detection in the visual editor (supported in FF...

Improve the image drag-resize detection in the visual editor (supported in FF and IE only), remove the size-* class if the image is soft-resized, see #18107

git-svn-id: https://core.svn.wordpress.org/trunk@18702 1a063a9b-81f0-0310-95a4-ce76da25c4cd