feat: render `.pdn` thumbnails.

[Sola-ris]

Summary

Add support for rendering .pdn thumbnails. Fixes https://github.com/TagStudioDev/TagStudio/issues/1147

.pdn files have an XML header that contains the thumbnail as a base64 encoded .png file. The file layout is the following:

• 4 byte magic number (PDN3)
• 24-bit little-endian int specifying the header's length
• The XML header
• The actual image data which we don't care about

The header itself looks like this:

<pdnImage width="800" height="600" layers="1" savedWithVersion="5.109.9343.2610">
    <custom>
        <thumb png="base64 string" />
    </custom>
</pdnImage>

Sources for the file format

• http://justsolve.archiveteam.org/wiki/Paint.NET_image
  • Apparently there's a compresed version of the file format, but I couldn't get Paint.NET to produce that one
• https://github.com/rivy/OpenPDN
  • https://github.com/rivy/OpenPDN/blob/master/src/Data/Document.cs#L64-L120
  • https://github.com/rivy/OpenPDN/blob/master/src/DocumentWorkspace.cs#L2594-L2610
• https://github.com/addisonElliott/pypdn/blob/master/pypdn/reader.py#L142-L143
• The test file opened in a text editor

Before

After

File used for testing

pdn.zip (The actual file is archived since GitHub doesn't allow uploading .pdn files)

Tasks Completed

• Platforms Tested:
  • [ ] Windows x86
  • [ ] Windows ARM
  • [ ] macOS x86
  • [ ] macOS ARM
  • [x] Linux x86
  • [ ] Linux ARM
• Tested For:
  • [x] Basic functionality
  • [ ] PyInstaller executable