Change image captions to semantic markup

Changing the image editor button to use semantic captions. This removes the need for caption.js loaded all the time.

### Issue
Currently, we load caption.js on almost all views of com_content, only to convert the title of an image to a caption if needed.

### Solution
HTML5 has a special tag for this captions.

### What I did
* This PR will change the image editor button to paste proper HTML5 tags into the article. If a caption is set, it will use `<figure><img ...><figcaption>abcd</figcaption></figure>`, if no caption is set, only the `<img ...>` tag is used.
* The form now also allows to set the title and caption independant, changing the caption field from a Yes/No list to a text field

### Testing
Insert images into an article using the image button below the editor. Make sure everything behaves as expected especially with and without caption.