NmtNewtForm is an NmtNewtWidget, but previously it was only realizing
its child, not itself, which is technically wrong (though it had no
noticeable effect until get_realized() was added).
Make all the signals RUN_FIRST instead of RUN_LAST. In particular,
this fixes a crash when creating a new connection and activating the
listbox directly, in which case nmt_newt_listbox_activated() was
running after NmtAddConnection:create_connection() had already quit
the form and unrealized the listbox.
