FileDialog: Always set transient

Otherwise dialogs could be garbage collected
3 jobs for master in 3 minutes and 27 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #15318
run-appdata

00:00:07

passed #15317
run-tests

00:03:08

 
  Build
passed #15319
run-build

00:00:11