_Widget: rename '_width' to '_length'
_Widget: rename '_width' to '_length'

Addresses #573

flacjacket/qtile,frostidaho/qtile,soulchainer/qtile,de-vri-es/qtile,aniruddhkanojia/qtile,aniruddhkanojia/qtile,himaaaatti/qtile,kseistrup/qtile,nxnfufunezn/qtile,qtile/qtile,kseistrup/qtile,farebord/qtile,nxnfufunezn/qtile,frostidaho/qtile,rxcomm/qtile,cortesi/qtile,dequis/qtile,kynikos/qtile,xplv/qtile,tych0/qtile,zordsdavini/qtile,andrewyoung1991/qtile,soulchainer/qtile,jdowner/qtile,cortesi/qtile,himaaaatti/qtile,StephenBarnes/qtile,de-vri-es/qtile,w1ndy/qtile,apinsard/qtile,dequis/qtile,tych0/qtile,qtile/qtile,apinsard/qtile,ramnes/qtile,ramnes/qtile,StephenBarnes/qtile,w1ndy/qtile,rxcomm/qtile,jdowner/qtile,xplv/qtile,kynikos/qtile,zordsdavini/qtile,kopchik/qtile,andrewyoung1991/qtile,kopchik/qtile,farebord/qtile,flacjacket/qtile