refresh caption and overlay info whenever the current photo updates

[zadr]

this allows for building more interactive captions by updating the view model and then having photosViewController(_:captionViewFor:) called again.

[cdzombak]

Thanks for the PR, @zadr! I have two questions/issues with this PR as it stands:

• How is this refresh triggered? As far as I can tell it happens when -updateImageForPhoto: is called, which isn't a particularly clear API (the docs for that method say, "Update the image displayed for the given photo object").

• When displaying multiple photos, this will cause the overlay view to flicker or reset for all photos, not just the one that changed. See for example this gif from the Example app, where the scrolled caption view is reset when a different photo is updated:

I am currently trying to make the dataflow for this library clearer (by, among other things, exposing the data source; see #163) which I hope will provide groundwork to add flexibility like this.

[zadr]

@cdzombak Hm. As far as your first point goes, I think it's relatively clear to say that if you call updateImageForPhoto:, it updates everything as required for the view to show the latest information.

If I'm understanding you correctly, the problem you're worried about boils down to updateImageForPhoto: being a blanket refreshing of all data (something changed, a la reloadData), rather than being able to diff changes between immutable photo models (this one thing in the backing data changed, a la reloadItemsAtIndexPaths:)?

This kinda thinking about view models really lends itself better to Swift and immutable structs than it does to Objective-C and mutable objects.

Anyway, moving to a data source-y basis does make it easy to ask for individual components on a per-photo basis, rather than having it all lumped together and having to diff a model object (or not being able to diff at all if the same underlying object is mutated), and that definitely seems like a decent direction to move in to me.

As far as the second point goes, oops. thats a bug. Will update the PR; thanks for catching it!