use gtk modelfilter for roster's treeview
we should fill when we get roster the model, and then show only those not offline by default using gtk's modelfilter
this allows us to be super fast when toggling show offline contacts (atm we go to draw all roster from scratch)
also overcomes a bug of GTK 2.8 (well not a reason but will they ever fix? etc..). see #1200 (closed), #1040 (closed)