main: Fix GApplication Open() not working

The playlist signals weren't correctly reconnected after being blocked to clear the playlist.

To reproduce manually:

totem --gapplication-service
gapplication launch org.gnome.Totem.Devel ~/Videos/foo.mp4

Bisected by Sebastian Keller

Fixes: 2a7952a7 ("main: Use GSignalGroup to simplify playlist signal handling")