gtk_accelerator_get_default_mod_mask() behaves differently on OS X under
GTK 3 when compared to GTK 2. On GTK 2 it used to clear the GDK_MOD2_MASK
bit while on GTK 3 it's preserved. We need to clear it ourselves
otherwise e.g. <Command>S leads to <Commands><Mod2>S and none of the
keybindings work under GTK 3.