Tweaked handling of inactive languages, deprecated font patching option, added TinyMCE support to LocalizeThis tool.

Inactive languages have been fixed so that they show up for filtering but not as an option for Default Language in the Translation settings page.

Also figured out how to get the LocalizeThis utility and TinyMCE to play well together. This partly involves patching the Editor.save() method, but it shouldn't cause any issues in terms of other plugins extending it the same way.

Also, since WordPress uses system fonts since 4.6, the Patch Font Stack option is hidden and won't be implementable on installs newer than 4.6.