Ad-hoc commands/dataforms: show list-single field as tree view instead of drop down menu when it is the only editable field

With this MR, when a data form only has one editable field of type list-multi list-single, we don't render it as a drop-down menu but a tree view instead. This improves usability quite a lot, since a form that has a single drop down menu is a bit weird and does not take advantage of screen real estate.

Screenshot from prosody "globally reload module" command:

Before:

After:

Screenshot from a slidge command:

Before:

After:

I think the code is OKish except https://dev.gajim.org/nicoco/gajim/-/commit/7d7d08ac598f9fdd2fdfc67160aa30f4958091fc which implements "trigger next stage by double clicking on a cell" which does not feel right. If the general idea of the MR is accepted, I'd be happy to get some guidance on how to properly "bubble up" the double-click event back to the "ad hoc command step page".

Besides that, I noticed dead code which I removed in https://dev.gajim.org/nicoco/gajim/-/commit/5714b47761f87a118370476a8706b97a34330a1c and a type in a class name fixed in https://dev.gajim.org/nicoco/gajim/-/commit/25c97091bf94c07c8b1c141e53c782f3bbe04706