Allow to set notification category

This backports upstream commit 791218a5 to set the notification category. In practice it changes the ABI so things built against PureOS might not run unmodified e.g. on Debian stable due to the new symbol (if a package uses this function) but i don't think we can do much about it and it likely doesn't matter much. We can reevaluate if we want to switch to glib 2.70 (or 2.72 since we have more upsteam changes pending) at a later point.

@matthias.klumpp @jeremiah.foster we discussed this before but a short ack would be nice for this core library.