I think it makes most sense to be able to add a layout template in source code view, but it's exactly in that view where the icon fades.
Unless you're starting on a clean article, adding a layout template adds it into the element the cursor is on.
(I dropped a thumbnail template into an article and each line of code was wrapped in a P tag.)
It should go above or below.
The only possibilty I had was to go to UIKIT an dcopy the structure from there and paset it into my source code where I wanted it.
Another advantage is that I can build up a page using layout parts even in existing content.