You can do this with CCSM or Compizconfig-settings-manager.
Install it with this command.
sudo apt-get install compizconfig-settings-manager
Open it and enable Window Rules plugin under Window Management section. Go to it's settings, On the Matches tab, There will options for settings various types of rule for windows.
Use the option
Above there. You have to add the window type to it. Then the windows of that class will always be above others.
For the dialog window, you need to add
type=Dialog in the above field. Like this
You can even use the + button to grab window class or type. Click on the icon, on the new window, Click on Grab button. The pointer will now become a crosshair icon. Click on the window you want to set rule for.
Experiment with the class and types to find your exact match.