Provide option for changing credential dialog position

Submitted by Karl

Assigned to Sean Davis @bluesabre

Link to original bug (#16459)

Description

I'd like to be able to change the default position of the dialog to enter the password when the screen is locked.

Reason: some monitor vendors like Dell decided to show a central nag-screen when the monitor resumes from stand-by or is turned on and the computer content is already shown. This nag-screen is hiding the underlying login screen which results in users that have to wait until the nag-screen disappears in order to see the login dialog. The nag-screen can't be turned off in the monitor settings. Even when you are used to typing without looking at the screen, you'll never know whether or not the key you have pressed to wake up the display/computer was written as the first character into the password enter field, resulting in a failed log-in.

A proposed solution would be the option to change the location of the dialog just like LightDM is using with the option "position" in "/etc/lightdm/lightdm-gtk-greeter.conf" as mentioned on https://askubuntu.com/a/434978

960px from the left and 25% from the top

position=960 25%

or 25% from the right and 400px from the bottom

position=-25% -400

My setup: xfce4-screensaver-command 0.1.8 xfce 4.1.4 Ubuntu 19.10

Version: 0.1.8