There is indeed a lot of work in here.
I am mobile, but I have been able to browse through part of the code. I have seen several things I have liked a lot, such as relying on templates for the property lists, or having a touched boolean property flag for groupextensions.
There are some modifications in App that will require some time to fully understand what is going on (and a proper screen).
I was not extremely enthusiastic to see extensive substitution of command instructions with the macro "FCMD_OBJ_CMD2". I read it like "freecad_command_object_command_two". I am by no means against reducing the typing and the footprint using a macro. It is probably that I do not get the name and thus the original command was clearer to me when reading the code despite its length. In any case, this is a non-issue probably falling within the realm of "matter of taste". Probably others will have differing opinions
I will try to take a deeper look when I have a real screen.
Thanks for taking the time to prepare this PR.