bindings: add a dedicated keycode for <Tab> for when a region is marked

In this way the keycode cannot be unbound from the 'indent' function,
so pressing <Tab> on a marked region will always indent the region.

This fixes https://savannah.gnu.org/bugs/?56960.

Bug existed since the "tabbing" of a marked region was introduced,
in version 2.9.2, commit 09958ebd.

src/global.c

@@ -1146,7 +1146,7 @@ void shortcut_init(void)
 	add_to_sclist(MMAIN, "M-6", 0, copy_text, 0);
 	add_to_sclist(MMAIN, "M-^", 0, copy_text, 0);
 	add_to_sclist(MMAIN, "M-}", 0, do_indent, 0);
-	add_to_sclist(MMAIN, "Tab", TAB_CODE, do_indent, 0);
+	add_to_sclist(MMAIN, "Tab", INDENT_KEY, do_indent, 0);
 	add_to_sclist(MMAIN, "M-{", 0, do_unindent, 0);
 	add_to_sclist(MMAIN, "Sh-Tab", SHIFT_TAB, do_unindent, 0);
 	add_to_sclist(MMAIN, "M-:", 0, record_macro, 0);

src/nano.h

@@ -605,6 +605,9 @@ enum
 #define SHIFT_DELETE 0x45D
 #define SHIFT_TAB 0x45F
 
+/* A special keycode for when <Tab> is pressed while the mark is on. */
+#define INDENT_KEY 0x4F1
+
 #ifdef USE_SLANG
 #ifdef ENABLE_UTF8
 #define KEY_BAD 0xFF  /* Clipped error code. */

src/winio.c

@@ -658,12 +658,8 @@ int parse_kbinput(WINDOW *win)
 
 #ifndef NANO_TINY
 	/* When <Tab> is pressed while the mark is on, do an indent. */
-	if (retval == TAB_CODE && openfile->mark && currmenu == MMAIN) {
+	if (retval == TAB_CODE && openfile->mark && currmenu == MMAIN)
-		const keystruct *command = first_sc_for(MMAIN, do_indent);
+		return INDENT_KEY;
-
-		meta_key = command->meta;
-		return command->keycode;
-	}
 #endif
 
 	switch (retval) {