desktop-integration: check owner of bus name
desktop-integration: check owner of bus name

It avoids calling D-Bus methods when the bus name
"org.gnome.SessionManager" does not exist.

Silences:
GSpice-WARNING **: Error calling 'org.gnome.SessionManager.Inhibit': GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.gnome.SessionManager was not provided by any .service files

fgouget/spice-gtk,Fantu/spice-gtk,fgouget/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,Fantu/spice-gtk,elmarco/spice-gtk,guodong/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,elmarco/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,SPICE/spice-gtk,elmarco/spice-gtk,guodong/spice-gtk,Fantu/spice-gtk,elmarco/spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,flexVDI/spice-gtk,Fantu/spice-gtk,SPICE/spice-gtk