Enable import/export of file formats other than .opossum

[benedikt-richter]

The goal of the current issue is to gather ideas on how to improve user experience by easing the import and export of data into the OpossumUI as well as by integrating more with existing tools, like scancode.

Some ideas include:

• directly open scancode/ORT jsons (however, for ORT, there is also an Opossum reporter)
• spdx import and export (related https://github.com/opossum-tool/opossum.lib.py)
• several files can be opened (all are merged in the frontend)

There is also current tooling for merging and converting several files: https://github.com/opossum-tool/opossum.lib.hs

[mstykow]

Similar to #1485.

[mstykow]

Similar to #113.