[Feature Request]: Thumbnails for PDFs

[Heiholf]

Checklist

• I am using an up-to-date version.
• I have read the documentation.
• I have searched existing issues.

Description

Render first page of PDF as thumbnail.

Solution

I can implement it myself, I just wanted to have a proper Issue so that the PR is not completely random.

Qt has PDF rendering functions, I would use those to prevent adding additional dependencies.

Alternatives

There might be faster PDF engines than the builtin in Qt. However, the speedup won't be gigantic and it is easier to use less dependencies.

[CyanVoxel]

PDF previews added in #543 (port of #378)