shell: Use GtkFileChooserNative for opening and saving files.

Replace GtkFileChooserDialog with GtkFileChooserNative for opening
and saving files, and save attachments.
Status Job ID Name Coverage
  Build
passed #745073
flatpak
flatpak

00:12:16

manual #745070
allowed to fail manual
master
passed #745071
meson

00:01:35

manual #745072
allowed to fail manual
meson-gtk-doc