Add volatile parameters.

Currently, a runtime change of a setting value is ignored, if the value is the same as the current value. In general that is what is wanted, but if the parameter refers e.g. to a file from which the actual setting value is read, there is no way to refresh the setting except by actually changing the file name.

There should be a way to mark a setting as volatile, which would cause to the setting to be "changed" even if the value is the same, which would then provide a way for additional activity.