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/settings_gui.cpp

@@ -1478,6 +1478,7 @@ static SettingsContainer &GetSettingsTree()
 			graphics->Add(new SettingEntry("gui.zoom_max"));
 			graphics->Add(new SettingEntry("gui.smallmap_land_colour"));
 			graphics->Add(new SettingEntry("gui.graph_line_thickness"));
+			graphics->Add(new SettingEntry("gui.show_restricted_signal_default"));
 		}
 
 		SettingsPage *sound = main->Add(new SettingsPage(STR_CONFIG_SETTING_SOUND));