Provide a plugin that will replace code entered in source with fake image in wysiwyg

[Jakub Ś]

Based on #7822.

This is very similar to proctectedSource.

1. Based on some regex, code that is to be hidden should be recognized.
2. When switching to wysiwyg this code should be replaced by image <code>...</code> (just like with flash).
3. When switching to source this code from image should be changed back to readable code.

Please note that it is impossible to edit such code but it is still possible to remove it by removing the image.

[Jakub Ś]

#4339 was marked as duplicate.

[Matthew Lieder]

FYI, I've implemented something similar to this as a plugin: ​http://ckeditor.com/addon/showprotected

Also, this ticket might be a dup of #851 (they seem pretty similar)