[TASK] RTE: Remove dependency on class Ext.Observable

RTE classes Editor and CSS.Parser extend Ext.Observable.
Since event handling was migrated to jQuery, this is not necessary anymore.

Releases: master
Resolves: #63654
Change-Id: I6c9287ea7f1dc0e785571c98f8fb604a2a7f959a
Reviewed-on: http://review.typo3.org/35150
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>