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/include/UI_interface.h

@@ -2558,6 +2558,7 @@ typedef struct uiDragColorHandle {
 
 void ED_operatortypes_ui(void);
 void ED_keymap_ui(struct wmKeyConfig *keyconf);
+void ED_uilisttypes_ui(void);
 
 void UI_drop_color_copy(struct wmDrag *drag, struct wmDropBox *drop);
 bool UI_drop_color_poll(struct bContext *C,