Reorganize Capture Options dialog

Opening Capture Options dialog on low resolution displays (e.g. 1366x768) in GNOME-shell makes user unable to see the bottom of the dialog

Change-Id: Icc39d5ca89f98edb3ff246cd23ab9663a3dfbd1e
Reviewed-on: https://code.wireshark.org/review/453
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>