Prevent leaking ThunarFiles when closing thunar while thumbnailing is in process

Version information

Thunar 4.21.1

Steps to reproduce

• Build thunar with meson setup --reconfigure build --buildtype=debug
• Stop thunar thunar -q
• Backup changed thumbnails mv ~/.cache/thumbnails ~/.cache/thumbnails.backup
• Enable thumbnailing in thunar
• Open a folder with many pictures (I tested with 400)
• Scroll down quickly, so that a thumbnail request for all pictures in the folder will be sent
• Quickly close thunar before thumnailing finished

Current behavior

• Message: --- Leaked a total of 210 ThunarFile objects: ....

Expected outcome

No ThunarFiles should be leaked

Additional information

The thumbnailing code was drastically changed in !416 (merged). Now every ThunarFile is holding a ThunarThumbnailer reference which is released when the file is finalized. However, the file cannot be finalized while its thumbnail request is not finished yet. The current code simply does not cancel thumbnail jobs on exit.

No fix, but some code changes: code.diff