React to close/save actions found in PreferencesDialog

[aalmiray]

I have the need to find out if the Close or Save buttons found in PreferencesDialog were clicked, but it's not possible to reach the event source (the buttons) without resorting to reflection hacks.

Suggestion: return the ButtonType in show(boolean) (see #7). However this suggestion does not cover the use case where the dialog is non-modal (old behavior of preferencesfx.show()) thus perhaps adding close/save event handlers (or listeners) to PreferencesFX would be better.