dbusServices: Allow to inhibit auto-shutdown
While we only shut down after a method call completed or (if the interface has signals) the sender disconnects from the bus, services may need to inhibit auto-shutdown for more specific reasons themselves, for example when a method call kicks off an operation that should complete before shutting down.
Add hold() and release() methods like Gio.Application for those cases.
Edited by Florian Müllner