The image caption checkbox is proposed even when html5 is not supported

Reproduction steps

• Start a 13.10.4 (i.e., a version not supporting html5 and with ckeditor 1.61)
• Open an editor
• insert an image
• search for a "captioned image" checkbox
  

Expected result:
The checkbox is only proposed when html5 is proposed

Actual result:
The checkbox is proposed

Consequences:
When inserting an image, the caption field is proposed on the widget and can be filled.
The text is inserted as a paragraph below the image