Added a setter for group.layout
Added a setter for group.layout

Allows both strings (with the layout name) and layout objects (that are
not required to be in the global layout list - not tested, though).

Also added a "layout" parameter to __init__, so you can do things like
[Group("a", "tile"), Group("b", "stack")] to set the initial layout.

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