region: Avoid a crash on panel destruction

GtkListBox might emit the row-selected signal while being destroyed
and our row-selected handler accesses widgets owned by GtkBuilder so,
depending on the order on which the builder destroys its objects, we
could crash on the signal handler when destroying the builder.

Using g_signal_connect_object() avoids this issue by disconnecting us
before our finalize() runs.

https://bugzilla.gnome.org/show_bug.cgi?id=770563