remove duplication from string constants

[StephanEggermont]

onTodoItemTap(item) {
const index = this.todos.indexOf(item);

action("What do you want to do with this task?", "Cancel", ["Mark completed", "Delete forever"]).then(result => {
    console.log(result);
    switch (result) {
        case "Mark completed":
            this.dones.unshift(item);
            this.todos.splice(index, 1);
            this.activeTabIndex = 1;
            break;
        case "Delete forever":
            this.todos.splice(index, 1);
            break;
        case "Cancel" || undefined:
            break;
    }
});

},