group and previous_group aren't class attributes

they're instance attributes, so they should be initialized that way.
Fix broken wlan and she widgets.

These widgets needed further re-factoring following commit
409130bb1a6bc5e93d44cd2f302bb1acfcb90716
fallback to en_US locale if no pair for default currency

Closes #429
[doc] add missing stack layout parameters

Closes #431
[stack layout] Update win order to mimic xmonad

 Stack layout now use the same order as monadtall or treetab.
 They can now be used in conjonction using the same keybindings.