Change focused field to paper size

When defaulting to destination folder, any (accidental) keyboard presses
would mess things up. Changed to paper size as the main thing that we
often want to change, not that this has any real impact when the dialog
is operated via touchscreen.

scan-dialog

@@ -112,7 +112,7 @@ function run_scan {
 export -f run_scan # Make function available to yad button command.
 
 # Display scan-dialog.
-yad --form --orient=hor --maximized --width=750 --height=250 --columns=3 \
+yad --form --orient=hor --maximized --width=750 --height=250 --columns=3 --focus-field=5 \
   --title="Scanning Options" \
   --field="Scan Folder:SFL" /mnt/public/scans \
   --field="Filename Prefix:SFL" scan \