[WIP][FEATURE] Allow continuous content record numbers

The CONTENT and the RECORDS content objects get a new setting called
continuousRecordNumbers.

If this setting is enabled the record counter in child content objects
will not be reset but continue counting. The parent content object
will then continue counting at the final count of the child content
object.

WIP: To have a clean counting it might be useful to disable the counter
for child CONTENT and RECORDS content object. In this way it would be
possible to use a even / odd class correctly. This setting can either
simply depend on continuousRecordNumbers or can also be an additional
setting (e.g. excludeContentObjectTypesFromCounting).

Resolves: #53717
Releases: 6.2
Change-Id: I3d1f9da6a12b9eb8d631350689f5a7c1f7ace3e5
[TASK] Improve code style of RelationHandler class

* add missing class properties, that are used in the code
* fix invalid PHPDoc variable types
* remove unused variables
* fix or remove invalid return values
* fix invalid variable types
* make sure unused variables are initialized

Resolves: #53718
Releases: 6.2
Change-Id: I90c1b73d0a4a7bb4666679e772805460786c805d