UI: Support defining UI lists in C

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.

source/blender/editors/space_api/spacetypes.c

@@ -129,6 +129,8 @@ void ED_spacetypes_init(void)
 
   ED_screen_user_menu_register();
 
+  ED_uilisttypes_ui();
+
   /* Gizmo types. */
   ED_gizmotypes_button_2d();
   ED_gizmotypes_dial_3d();