SSRPlugin 0.2 - minor changes

After Trond's e-mail I've changed slightly the behaviour of SSRPlugin. When Requestor cancels its request, he is not supposed to provide the reason (appropriate entry in SSR schema looks like that: <xsd:element name="Cancel">, which implies that <Cancel/> element should be empty). I still keep a pane for `Cancellation reasons' when Acknowledging station denies the request - it will display the message informing the Requestor that his request was denied.