RTE: Mark CSS class as not selectable
As proposed in #51905, to improve dependencies management between classes, we can mark a class as not selectable.
If it's not selectable, it's not displayed inside blockstyle, textstyle plugin and other style selection.
When checking dependency after removing another class, if this class is setted on the element and is not required for another dependencies, it also remove this class to avoid orphan unselectable classes which can't be removed with select styles.
Outside #51905, it's a pretty useless feature.