Make sure signals and hooks are all properly disconnected at deletion of Oxygen::Style::Instance

Delete TimeLine::Server in theme_exit.

Addresses (partially) https://bugs.mageia.org/show_bug.cgi?id=2679
To my understanding, there is still a crash due to DBus (still working on it),
and issues with gtk_window_set_composite (reverting to old value does not seem to work. Might be a GTk bug).

CCBUG: 275366