Improve the behaviour of the pageNumber input field

This PR adds the following two improvements to the pageNumber input element:
* When the user clicks on the pageNumber input element, the content is selected.
* If a floating point number is entered, it's handled similarly to other pdf viewers. In this case, pdf.js will no longer fail @[web/viewer.js#L826](https://github.com/mozilla/pdf.js/blob/master/web/viewer.js#L826) because `val` is a float.