Fix T85223: Some modifier panels can dissapear in old files

The problem is that each uiBlock needs to be assigned a unique name,
but when there can be multiple modifiers of the same type, we use the
panel sort order for the unique part of the string. However, the most
recent test file has 1200+ panels in the property editor, so 4
characters isn't enough for a unique string.

That's not a situation I expected, but it makes sense, because we don't
remove legacy panels with unused types when loading old files. So they
tend to accumulate a bunch of unused panels. That's why this works fine
with a new property editor, it doesn't all of the extra old panels.

These panels must be stored for the expansion status and order, but
arguably we could cull unused panels on save. However, simply increasing
the length of the unique panel string is a valid fix in this situation.
In the future, we can look into removing unused panels when saving.

source/blender/editors/include/UI_interface.h

@@ -1729,7 +1729,7 @@ struct Panel *UI_panel_add_instanced(const struct bContext *C,
                                      struct PointerRNA *custom_data);
 void UI_panels_free_instanced(const struct bContext *C, struct ARegion *region);
 
-#define INSTANCED_PANEL_UNIQUE_STR_LEN 4
+#define INSTANCED_PANEL_UNIQUE_STR_LEN 16
 void UI_list_panel_unique_str(struct Panel *panel, char *r_name);
 
 typedef void (*uiListPanelIDFromDataFunc)(void *data_link, char *r_idname);