gtkplacesview: make consistent loading state

gtkplacesview: make consistent loading state

Until now the code was not very clear about why the loading property is
needed, since we didn't forced all the async operations to mark the
view as loading. This cause that clients are not aware when the view
is busy on those situations.

For instance Nautilus uses the property for a few things, one of it
is to show a busy spinner on the tab title.

To improve the situation, mark as loading when a volume operation,
a mount operation or a connect to server operation is being performed.

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