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.