Scheduled dispatch: Add clear schedule GUI function

Add management dropdown box to window

src/schdispatch_cmd.cpp

@@ -276,6 +276,36 @@ CommandCost CmdScheduledDispatchResetLastDispatch(TileIndex tile, DoCommandFlag
 	return CommandCost();
 }
 
+/**
+ * Clear scheduled dispatch schedule
+ *
+ * @param tile Not used.
+ * @param flags Operation to perform.
+ * @param p1 Vehicle index
+ * @param p2 Not used
+ * @param text unused
+ * @return the cost of this operation or an error
+ */
+CommandCost CmdScheduledDispatchClear(TileIndex tile, DoCommandFlag flags, uint32 p1, uint32 p2, const char *text)
+{
+	VehicleID veh = GB(p1, 0, 20);
+
+	Vehicle *v = Vehicle::GetIfValid(veh);
+	if (v == nullptr || !v->IsPrimaryVehicle()) return CMD_ERROR;
+
+	CommandCost ret = CheckOwnership(v->owner);
+	if (ret.Failed()) return ret;
+
+	if (v->orders.list == nullptr) return CMD_ERROR;
+
+	if (flags & DC_EXEC) {
+		v->orders.list->ClearScheduledDispatch();
+		SetWindowDirty(WC_SCHDISPATCH_SLOTS, v->index);
+	}
+
+	return CommandCost();
+}
+
 /**
  * Set scheduled dispatch slot list.
  * @param dispatch_list The offset time list, must be correctly sorted.