Period field doesn't show multiple period values in JFR Wizard

When configuring the event settings for a flight recording and selecting a folder in the tree where the events selected have differing period values the text box becomes empty. This may lead the user to believe that there is no period set for those events, and write one that overrides all of them instead. We should show that this is not the case, preferably by using a similar method of displaying multiple values as we do in the Properties sheet in JFR. I.e. [Once every chunk, 10ms, 1000ms] et c.