Replaced Disabled Attribute With Readonly

This has the advantage that one can listen to click events on buttons, although they are "disabled". Click events are NOT fired on inputs with the `disabled` attribute, but on inputs with the `readonly` attribute. [The differences between the `disabled` and `readonly` attributes](https://stackoverflow.com/questions/7730695/whats-the-difference-between-disabled-disabled-and-readonly-readonly-for-ht) are, apart from that fact, rather marginal.

In practice, this enables people e.g. to implement a full carousel, which also continues when the previous button is clicked on  the first element.