Initial implementation of trace restrict slots

src/window_type.h

@@ -687,6 +687,12 @@ enum WindowClass {
 	 */
 	WC_TRACE_RESTRICT,
 
+	/**
+	 * Trace restrict slot window; %Window numbers:
+	 *   - Packed value = #SlotListWidgets / #VehicleListWidgets
+	 */
+	WC_TRACE_RESTRICT_SLOTS,
+
 	WC_INVALID = 0xFFFF, ///< Invalid window.
 };