Improve start-up performance

[saarmbruster]

Adding the pages to the Scene Graph the first time they are accessed instead when initializing the CalendarView improves the start-up time significantly, especially if a stage contains multiple instances of the CalendarView. Only the day page must be added at startup because it's the default page.

In my case there is a stage with a TabPane that contains multiple CalendarView instances. Without this change the application freezes for about 10-20 seconds until the window is shown. With this change the window is already shown after less than one second.

The only noticeable drawback of this change is a minimal delay (~1-2 seconds) when switching to the sheet view in the year page for the first time.

[dlemmermann]

Does this still work nicely with the animations (zoom in / out) when switching between the views?

[saarmbruster]

At least on my machine it does. Only the year page animation might sometimes look a bit shorter for the first time you click on the year button. But once the page has been added there is no difference anymore.

[dlemmermann]

I actually did the same change in another project of mine. Makes total sense.

[dlemmermann]

I just ran the HelloCalendarView sample which does not use animation to switch between pages. In this sample the week, month, year pages do not show up anymore. I think this might be related to your change. Could you please look into this?

[saarmbruster]

Sure, I will check this.