Add option to always ask where to save downloaded files

Fixes: GNOME/epiphany#147
3 jobs from ask-download in 20 minutes and 22 seconds
latest
Status Job ID Name Coverage
  Test
passed #185902
flatpak

00:18:21

 
  Deploy
passed #185903
review

00:02:00

manual #185904
allowed to fail manual
stop_review