groundwork for an interface change

Add a new window-port interface function
     perminvent_info *
     update_invent_slot(winid window, int slot, perminvent_info *);

That should be nice and flexible and allow exchanges of useful
information between the core and the window port. Information
to be exchange can be easily modified in include/wintype.h as
things evolve.

Information useful to the core can be exchanged from the
window-port in struct to_core.

Information useful from the core to the window-port can be
passed in struct from_core.

I'm not going to update any docs until much later after things
are fully working and settled.

This also doesn't fix or have anything to do with existing
TTY_PERM_INVENT issues.

include/wintype.h

@@ -162,6 +162,28 @@ typedef struct gi {
 
 #define MENU_BEHAVE_STANDARD      0x0000000U
 
+struct to_core {
+    boolean active;
+    boolean use_update_inventory;    /* disable the newer slot interface */
+    int slotcount;
+    int low_slot_num, high_slot_num;
+    int max_slot_text;
+};
+
+struct from_core {
+    long piflags;
+    int slot;           /* which inventory slot; 0 means info exchange only */
+    int invlet;
+    const char *text;   /* the text to display */
+};
+
+struct perminvent_info_t {
+    struct to_core tocore;
+    struct from_core fromcore;
+};
+
+typedef struct perminvent_info_t perminvent_info;
+
 /* clang-format on */
 
 #endif /* WINTYPE_H */