BUG:344344 Don't show the splash until the window is open on Gnome

Gnome 3's window manager and Qt's splash screen don't work together.
Scribus, Karbon, Krita -- every application with a Qt splash shows an
empty rectangle until the main window is mapped. Is it a Qt bug? Is
it a Gnome bug? I don't know, but it looks bad, so on Gnome, don't splash
until the mainwindow is shown.