API: add getPageCount() method

[ronja-koistinen]

Our designers have a need for displaying the total page count on the page. I figured out you can get it like this:

    tify = new Tify();
    tify.ready.then(() => {
        console.log(tify.app.config.globalProperties.$store.pageCount);
    });

But I thought it would be nice to have an easier to use method for this.