Feature: #58122 - Configure class as non-selectable in Rich Text Editor

See forge#58122


It is now possible to configure a class as non-selectable in the style selectors of the Rich Text Editor.

The syntax of this new property is

RTE.classes.[ *classname* ] {
	.selectable = boolean; if set to 0, the class is not selectable in the style selectors; if the property is omitted or set to 1, the class is selectable in the style selectors


There is no impact on previous configurations.