Refactor tag/value writer for new data model

[nicoweidner]

After the JSON writer was done in #359, the tag/value writer should follow next. See the linked ticket and the corresponding PR/changes to get an overview. The previous state can also serve as an example, see https://github.com/spdx/tools-python/blob/b8fde35e141daf082b8b0fe32f875da06e6316e6/spdx/writers/tagvalue.py

[armintaenzertng]

The tag-value writer has been merged already to provide the ability to implement synergies with the json writer. The specific points I encountered were:
#395 (comment)
#395 (comment)

[meretp]

This issue is fixed with #395. With #402 we have an open issue to make the tag-value writer and json writer consistent.