Update .gitignore

[f-brinkmann]

Does it make sense to inlcude these?

[HaHeho]

I also noticed that some exclusions were missing in the .gitignore when working on macOS.

I would add a slightly more complete lists of exclusions for the popular OSs:

# Linux
*~
.Trash-*

# macOS
.DS_Store
Icon
._*

# Windows
Thumbs.db
[Dd]esktop.ini

[HaHeho]

@tuil-emt, can we merge some of these PRs? My impression is that there are already so many versions of the code base scattered in different repositories. This repo could serve as an anchor for the latest implementations and experiments people were making. For that, it would be good to clean up the PRs so they get abandoned, as long as the people involved are around and have an active memory of their intentions with the proposed changes.