[BUGFIX] Declare ``items`` for each RTE instance

The variable ``items`` was declared in a prototype instance and thus
shared between all RTE instances. Declare the variable in its own
instance instead.

Change-Id: Ia6f8b5dffb19121ed8995afcc2124b0084be5f7a
Resolves: #68392
Releases: master, 7.6
Reviewed-on: https://review.typo3.org/46982


Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Eugen Lang <laeu1011+gerrit@gmail.com>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>