Revert 0f805bfd (new default cycle_group keybinding to Alt-grave)

There are two wrong assumptions there:
- ` being above tab is surely true for some qwerty layout, but not in
  other layout like azerty (² is above the tab). We should find a clever
  way to find the key above tab
- ` is proceeded in azerty keyboard (french layout) by <Alt Gr + è>. The
  issue is that <Shift + è> is "7". And so, "Alt + 7" (used in some
  applications like irssi, weechat, …) is <Alt + Shift + è>, which is
  the reverse order to naviguate between grouped windows, and so, you
  can't anymore access to this keybinding.