[BUGFIX] Content rendering: Definition lists in RTE are wrapped in p

Definition lists entered in the RTE are wrapped in paragraph tags when
rendered in the frontend. This is not valid HTML.

Solution: Add dl and dd to lib.parseFunc_RTE.externalBlocks and
process them recursively like div. Add dt to
nonTypoTagStdWrap.encapsLines.encapsTagList

Resolves: #25083
Releases: 4.5, 4.6, 4.7

Change-Id: Ia73d01105beeaba59f91dd87754c7528796d4d07
Reviewed-on: http://review.typo3.org/4181
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch