Application/Script Launcher plugin

Please review my code. Currently, every "execute" action will be called twice. I don't know if that's a core issue or the bug is in the cmdmgr.py script. I can't find any bugs in the plugin so fare...
ActionParameterError is not suited here. Please add an error code in devicemanager: "DeviceErrorDeviceBusy" and use that. You should also update server/jsonrpc/actionhandler.cpp to handle that.