I tried your code on latest soft released Plone (5.1.1), got into troubles unrelated to your patch, but finally got it to run: well, it works.
However, there is a slight problem.
In Plone 4, there was a title and description and a caption checkbox.
In Plone 5, there is a title, an alt field and a caption checkbox. The title goes into Plone as, well, the field 'title'.
The alt 'field' does not, it's only a tag in the Tinymce html.
So the user after saving its work with TinyMce would have to search the image at whatever place it would be stored, edit it, add a description, save it. That's definitely not as easy as what was possible with Plone 4. Furthermore since saving the image can changes its URL the user would have to get back to the document, edit it, save it again.
To get at some parity with Plone 4, it would be necessary to add a description field to the dialog and save it into Plone when creating the image.
There is also to actually enable the captioning of course but for an experienced Plonista such as you it should be as easy as pie. Being the barbarian I am I just hacked savagely at Plone and got:
<dl class="image-inline captioned">
<dt><a rel="lightbox" href="/Plone/ftest/missfonts.png"><img src="http://ploneprod5:8083/Plone/ftest/missfonts.png/@@images/f70ef1e7-66fa-4faf-b368-414e9878cc51.png" alt="alternative text image 1" title="titleImage1" width="768" height="453"></a></dt>
<dd class="image-caption">description for missfonts.png</dd>
should be good enough for all but the most purists of html purists.