Fix typo in accessors for DashboardContext.dashboardPersister

While looking around the doc and project to get the hang on how to use it, I've been buggered by that typo (missing `s`) on accessors for `DashboardPersister` property in `DashboardContext`.

So here's a pull request to rename the offending methods (both in the class and where they are called in the project), while keeping the previous ones as *deprecated* in order to avoid breaking users' projects.

Those deprecated method with the typo should be removed later on.