widget: add main channel before other channels
widget: add main channel before other channels

Make sure that the d->main channel member can be referenced when
adding further channels, when we perform their setup.

Fantu/spice-gtk,mathslinux/spice-gtk,flexVDI/spice-gtk,dezelin/spice-gtk,elmarco/spice-gtk,mathslinux/spice-gtk,mathslinux/spice-gtk,fgouget/spice-gtk,guodong/spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk,flexVDI/spice-gtk,mathslinux/spice-gtk,dezelin/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,SPICE/spice-gtk,dezelin/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,elmarco/spice-gtk,Fantu/spice-gtk,guodong/spice-gtk,fgouget/spice-gtk,guodong/spice-gtk,Fantu/spice-gtk,SPICE/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,elmarco/spice-gtk,fgouget/spice-gtk,guodong/spice-gtk,Fantu/spice-gtk,dezelin/spice-gtk,elmarco/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk