Don't remove an existing app icon

This corrects commit 0d56bd9abd0976e18e8b1db8b80e6326e74edc10
which had the effect of removing the embedded application icon
on systems like Mac and MS Windows where icon themes are typically
not configured, and QIcon::fromTheme() thus returns an empty QIcon.
QIcon::fromTheme() should always be called with a valid fallback
icon, for instance the target icon (QApplication::windowIcon()).