MainWindow: fix "fudge factor" for the "header vs. logo"
MainWindow: fix "fudge factor" for the "header vs. logo"

I'm planning on redesign of the MainWindow after the next release, so
I'll stick with the "fudge" approach for now.

Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui