Refactor `CopyBuilder`s to use `thisInstance` to return itself.

[staudtMarius]

This is the behaviour we want:

but there are many cases, where we have the following code:

Therefore, all part should be adapted to be like the first example.

[sebastian-peter]

@staudtMarius Just to better understand this issue: What's the purpose of this? I think thisInstance was introduced out of necessity for those copy builders that were subclassed and still needed to return the right type. For copy builders that are not subclassed, using this should've been fine too.

[staudtMarius]

My idea was to harmonize the code.