TinyMCE wpView:
- Add loadingPlaceholder() that returns some html to use as placeholder while the view is loading. Includes a subtle CSS based loading animation.
- Fix setContent(), it should empty the element before appending the new node.
- Change getHtml() to always return a string.
Fixes #28761.
Built from https://develop.svn.wordpress.org/trunk@29019


git-svn-id: http://core.svn.wordpress.org/trunk@28807 1a063a9b-81f0-0310-95a4-ce76da25c4cd