validate(section_id, value) {
		return true;
	}
Apply custom validation logic.

This method is invoked whenever incremental validation is performed on
the user input, e.g. on keyup or blur events.

The default implementation of this method does nothing and always
returns `true`. User code may override this method to provide
additional validation logic which is not covered by data type
constraints.

@abstract
@param {string} section_id
The configuration section ID

@param {*} value
The value to validate

@returns {*}
The method shall return `true` to accept the given value. Any other
return value is treated as failure, converted to a string and displayed
as error message to the user.