Fix KCM reloading the plugins N times when a device was selcted

Every time a device is selected, pluginsConfigChanged would trigger even if no plugins actually changed (there's a boolean to distinghish that on the signal that we were ignoring). On top of that, we were connecting to the signal again each time we selected a device, causing this bug to trigger N+1 times every time a device was selected.

This is bad because pluginsConfigChanged would reload all the plugins, which for some plugins even implies network calls to the connected device.