[TASK] Mark several parts within RteHtmlParser as internal

RteHtmlParser has never received a good clean up, that's
because it was there since PHP 4 ages.

In order to rename, reorder code, all internal methods
and properties are now declared as protected.

Resolves: #86440
Releases: master
Change-Id: I204efa8156abcf9ae07a607fa03fa04fbb269885
Reviewed-on: https://review.typo3.org/58457


Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>