Allow stateful components with PermalinkProvider

[geographika]

Possible fix for #631.

Inherit from CookieProvider so other components in the application can save state, and only set map state when map object/value properties are passed.

This has the drawback that the LocalStorageProvider cannot be used, so perhaps redesigning the PermalinkProvider as a mixin to be added to any Provider class would be a better approach?

CC @leonardospina

[chrismayer]

Looks good at first sight. Couldn't test it yet. Feels like a (good) workaround. But we should think of a more general approach, as you already stated.

[geographika]

How about adding the current PermalinkProvider functions to a PermalinkMixin and then adding the mixin to 2 new GeoExt classes deriving from the 2 ExtJS state providers?

E.g. GeoExt.state.CookieProvider and GeoExt.state.LocalStorageProvider

[chrismayer]

Creating a mixin and two dedicated class definitions having the mixin sounds like a good idea. If that will work this would be my preferred way to solve this.

Should a third class GeoExt.state.PermalinkProvider also be available or do you think this is way too much? Think on uses cases where you don't want to use cookies or LocalStorage.

[geographika]

Should a third class GeoExt.state.PermalinkProvider also be available or do you think this is way too much? Think on uses cases where you don't want to use cookies or LocalStorage.

I think the mixin would be enough? The GeoExt.state.PermalinkProvider would be empty apart from the mixin, so probably easier to add the mixin to a custom class as and when required.