Add setting to show restricted electric signals using the default sprites.

This is to improve visual distinctiveness of restricted signals when
using signal GRFs.
This defaults to off.

src/table/settings.ini

@@ -3133,6 +3133,14 @@ strhelp  = STR_CONFIG_SETTING_GRAPH_LINE_THICKNESS_HELPTEXT
 strval   = STR_JUST_COMMA
 proc     = RedrawScreen
 
+[SDTC_BOOL]
+var      = gui.show_restricted_signal_default
+flags    = SLF_NOT_IN_SAVE | SLF_NO_NETWORK_SYNC
+def      = false
+str      = STR_CONFIG_SETTING_SHOW_RESTRICTED_SIG_DEF
+strhelp  = STR_CONFIG_SETTING_SHOW_RESTRICTED_SIG_DEF_HELPTEXT
+proc     = RedrawScreen
+
 ; For the dedicated build we'll enable dates in logs by default.
 [SDTC_BOOL]
 ifdef    = DEDICATED