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
mpris2widget.py:

            * A widget for MPRIS2 compatible players.
            * Uses scrolling text or like the old mpriswidget.py
            * No problems with DBus, it uses a DBus receiver
Fix for instantation of the widget. Uneeded super() call.
    * Usage of a class attribute as scroll counter
    * Indentation fixes
Drop unneeded import