[TASK] RTE: Remove dependency of plugin classes on ExtJS inheritance

Use minimal custom inheritance.

Releases: master
Resolves: #63693
Change-Id: Ic7f306ca0be6989a29f1a04a232d0f642807a560
Reviewed-on: http://review.typo3.org/35177
Reviewed-by: Stanislas Rolland <typo3@sjbr.ca>
Tested-by: Stanislas Rolland <typo3@sjbr.ca>