gtkplacessidebar: avoid accessing destroyed popover

gtkplacessidebar: avoid accessing destroyed popover

The row and rename popovers are always relative_to a row.
We also keep a pointer to them so we can interact with them in
callbacks.

However, if the row is destroyed its associated popovers will be
destroyed as well as relative_to destroyes and frees memory of its
associated widget when its relative_to widget is destroyed.
If we, for example, update the places while the popover is shown we are
going to access and invalid widget on the next time.

To avoid that, connect to the destroy signal of the popovers and clean
the sidebar pointers when that happens.

https://bugzilla.gnome.org/show_bug.cgi?id=755444