on wayland I'm getting a GDK_MOD1_MASK and a GDK_META_MASK
on pressing the left alt. The check for KEY_MOD2 in
Window::KeyInput trips up with the extra KEY_MOD3 bit set
so the auto mnemonic underlines don't appear on pressing
left alt under wayland

Lets map only GDK_SUPER_MASK to KEY_MOD3

Change-Id: I1e9cc9fc095f5edfa7ad7c71440232c6de1ecf04