Cleanup attachment save handling

Follow GTK+ reccomendation for (not) setting folder paths in save file
choosers, ensure UTF-8 display names and file system encodings are used
as appropriate, and report errors to the user when they occur.
4 jobs for wip/cleanup-attachment-save-handling in 12 minutes and 22 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #131697
deb-package

00:08:02

passed #131695
fedora

00:08:06

passed #131698
flatpack-package

00:12:20

passed #131696
ubuntu

00:06:46