Rate Limit updates client-side

KSystemStats used to run at an update rate of 2s, which was too slow and we got a few bug reports of. For 5.21 we changed the update rate to 500ms, so now we get bug reports about it being too fast. However, since the base rate is now fast enough, we can simply reduce the number of updates on the client side, to allow people to configure how fast things should update. This implements that, adding a configuration item to the Appearance page of plasmoids and ensuring all items in a face respect it.