Certain markup breaks the Source Editing mode

[psmyrek]

Suggested merge commit message (convention)

Fix (html-support): Skip inline image upcast conversion inside not supported element. Closes #10703.

[psmyrek]

It was quite difficult for me to find out what is going on here. Below there are some notes I was able to find out while debugging this problem:

• The DataFilter#_registerElementHandlers() registers converters based on the type of a given element. But if a given element is both an object and an inline type, then $text will not be extended with new attributes introduced by this element. Is this approach correct? Probably yes, as they will be stored in the htmlAttributes attribute, but it was a surprise to me a bit.
• Initially, I wanted to fix this issue in another way. If an element is an object and an inline type (there are several such elements, i.e. htmlImg), then we should additionally register attribute converters by calling the DataFilter#_registerInlineElement() for this element. This, however, would not be enough because the upcast converter from image integration is called before the default converter for inline elements, so the one from the image integration had to be reordered with 'low' - 1 priority. In the end, I decided that it is too complicated and the current fix in this PR seems to be enough.