notifications: Use g_signal_connect_object to ensure handlers are not run after object destroyed

Fixes specific crash:
GNOME/gnome-control-center#75
5 jobs for <span class="ref-name">notifications-signal-handler</span> in 6 minutes and 26 seconds (queued for 7 minutes and 49 seconds)
Status Job ID Name Coverage
  Build
passed #56696
build

00:01:49

 
  Test
passed #56699
allowed to fail
asan

00:01:46

passed #56698
coverage

00:02:06

passed #56697
test

00:00:43

 
  Delivery
manual #56700
allowed to fail manual
packaging