Allow setting direction to either ltr or rtl

Problem/Motivation

Currently this module only adds a single on/off button that toggles the [dir="rtl"] attribute on the selected element.
In the settings of the text format , we can also configure it to toggle the [dir="ltr"] attribute instead.

But that's not enough.

This module should have two buttons:
One button to toggle the [dir="rtl"] attribute
And another to toggle the [dir="ltr"]

We should be able to set the [dir] attribute to either "rtl" or "ltr" or remove it as needed.

(similar to what was available in Ckeditor 4)