Fix the check for VTE's notification-received patch

[egmontkob]

See https://gitlab.gnome.org/GNOME/vte/-/issues/2858 for details. Quick summary:

The investigator of the crash said

Ah, but terminator appears to be deliberately using this code as a feature test. [...] if the signal fails to connect, it falls back to assuming that that vte patch is unavailable. It's just that it doesn't cope if vte actually crashes.

To which VTE's main developer responded

Creating an instance and connecting to a signal to check if it exist isn't the right way to do this. The right way is to use g_signal_lookup with the name and gtype.

Although VTE was patched not to crash anymore, it would be nice if Terminator switched to the proper way of testing.