Initial implementation of tracerestrict counter mechanism

src/window_type.h

@@ -733,6 +733,12 @@ enum WindowClass {
 	 */
 	WC_TRACE_RESTRICT_SLOTS,
 
+	/**
+	 * Trace restrict counter window; %Window numbers:
+	 *   - Packed value = #SlotListWidgets / #VehicleListWidgets
+	 */
+	WC_TRACE_RESTRICT_COUNTERS,
+
 	/**
 	 * Programmable pre-signals window
 	 */