So far all UI lists had to be defined in Python, this makes it possible to define them in C as well. Note that there is a whole bunch of special handling for the Python API that isn't there for C. I think most importantly custom properties support, which currently can't be added for C defined UI lists. The upcoming asset view UI template will use this, which needs to be defined in C. Adds a new file `interface_template_list.cc`, which at this point is mostly a dummy to have a place for the `ED_uilisttypes_ui()` definition. I plan a separate cleanup to move the UI-list template to that file.