TinyMCE: image caption break if the user changes the "Block format"

[azaozz]

To reproduce:

• Select an image that has a caption.
• Change the Block format (the drop-down on the second toolbar row) to <p>, <h1>, <h2>, etc.

Happens because the caption "wrapper tag" is changed. This is generally a user error, but we can try to correct it.

[azaozz]

In 33477.patch: when converting images with captions from HTML elements to shortcodes, make the wrapper div optional.

This div is a left over. Still there for back-compat. It was used to make it possible to center images with captions in old IE where centering of block elements in contentEditable is broken.

[azaozz]

In 33834:

TinyMCE: fix replacing the image caption HTML with the shortcode when saving the content.
Fixes #33477.