Feature: Remember the last-used signal between games

(cherry picked from commit a9de36ad23f30d125ed495f0bd84342b594d19df)

See: https://github.com/OpenTTD/OpenTTD/pull/9792

src/table/settings/settings.ini

@@ -4935,6 +4935,14 @@ strhelp  = STR_CONFIG_SETTING_CYCLE_SIGNAL_TYPES_HELPTEXT
 strval   = STR_CONFIG_SETTING_CYCLE_SIGNAL_PBS
 cat      = SC_ADVANCED
 
+[SDTC_VAR]
+var      = gui.default_signal_type
+type     = SLE_UINT8
+flags    = SF_NOT_IN_SAVE | SF_NO_NETWORK_SYNC
+def      = 5
+min      = 0
+max      = 5
+
 [SDTC_VAR]
 var      = gui.drag_signals_density
 type     = SLE_UINT8