widget: add Configurable to *Mixin widgets
widget: add Configurable to *Mixin widgets

our doc generator looks for subclasses of configurable.Configurable in
order to figure out what to detect defaults from. let's add
configurable.Configurable to these classes so that their defaults also get
added to the list of widget options.

Closes #1545

Signed-off-by: Tycho Andersen <6b523322678240882acbb4da5312a12d8cd0082d@tycho.ws>

tych0/qtile,ramnes/qtile,qtile/qtile,ramnes/qtile,qtile/qtile,tych0/qtile