Fix "Shutdown" not working & fix NPE in Pictures

Shutdown uses the EventServer and this brings up the shutdown
dialog where you need to choose what to do. This behaviour is not
inline with what we experience on iOS client and makes it hard to
shut down a system from remote as the selection might end up in
dialog close or the first item. Convert the call to JSON-RPC
instead. This will gracefully shutdown the XBMC server and power
off the system