Fix "Running smartctl..." popup behavior #17

Reported by email:

The popup that says "Running 'smartctl' on /dev/sdx..." will float above
all other windows on the desktop even when they should be on top (e.g.
switching to Firefox while this popup is active, the popup will render
on top of the Firefox window.) Also, Gnome's greying out of the windows
behind the popup doesn't work properly. I suspect that the popup does
not have it's parent window properly set, which would cause these types
of issues.

Tested with GSmartControl 1.1.3 on Ubuntu 20.04.1.