gtkplacesview: don't rely on widgets on headers

gtkplacesview: don't rely on widgets on headers

We are showing a GtkSpinner on the networks header to provide feedback
to the user if we are fetching networks, therefore we have to modify
the spinner state when doing it.
However GtkListBox doesn't give guarantees about the widgets
set by gtk_list_box_set_header, and we could access an invalid
widget.

To avoid to access invalid widgets, bind the fetching networks
view property to the networks header spinner active property instead
of modifying directly the spinner in the private structure.

Not having the spinner in the private structure also makes the code
cleaner.

https://bugzilla.gnome.org/show_bug.cgi?id=754150