Currently there is no way to configure the network connections in systemsettings even if there is a section "Network". IMHO the attached window should be accessible through the systemsettings window (bluetooth and web (proxy, ssl, ...) settings are also there).
Created attachment 104147 [details] Systemsettings without network configuration
Created attachment 104148 [details] Network configuration window
There is a new KCM in Plasma 5.9.