box-config: Don't use object from closure
box-config: Don't use object from closure

Using the object in lambda from closure will add a ref on the object in
question and object will leak unless id of signal is saved and signal
handler is explicitly disconnected. Since signal handlers are passed the
source object, we can simply use that and avoid redundant ref.

https://bugzilla.gnome.org/show_bug.cgi?id=746800

vbenes/gnome-boxes,vbenes/gnome-boxes