Set window icon in code instead of in the desktop file

%i is expanded to "--icon xxx", but Qt only supports that option on
xcb, not Wayland. Therefore kget couldn't be started from the
application launcher/menu on Wayland.
See https://community.kde.org/Guidelines_and_HOWTOs/Wayland_Porting_Notes#Application_Icon .

Instead, use QApplication::setWindowIcon() to set the window icon
(necessary to have a window icon on other desktops than Plasma). This
also has the advantage that the proper window icon is set when running
kget from a shell (and not passing the --icon or -qwindowicon options).

BUG: 400367
FIXED-IN: 18.12.1