Add support for custom header views

[frosty]

This PR adds support for custom header views in the media picker collection view. This will be used in WordPress-iOS to show an info panel at the top of the picker if a user has only provided limited access to their photo library.

Three methods have been added to the media picker delegate protocol:

• mediaPickerController(_:configureCustomHeaderView:)
• mediaPickerControllerReferenceSizeForCustomHeaderView(_:)
• mediaPickerControllerShouldShowCustomHeaderView(_:)

These in combination allow a delegate to control whether a custom header view is displayed, configure the view if necessary before it's displayed, and provide a reference size which will be used to display the view at the correct size.

In order for these to have an effect, the custom header type must be registered using a new method on WPMediaPickerViewController: registerClassForCustomHeaderView(_:). The class must be a subclass of UICollectionReusableView.

I've added an option to the demo app to show an example custom header:

There are a few small caveats around the current implementation:

• Only one header is supported, and is displayed at the top of the collection view. Headers scroll with the collection view content.
• Because the 'capture cell' (displaying a live feed of the device's camera, allowing the user to capture an image) also uses a collection view header, it can't be used at the same time as a custom header.

To test

• Build and run the demo app
• Tap the + button and navigate around the picker. Ensure you don't see a special header above the regular image cells. Ensure you can select an image.
• Dismiss the picker and tap Options in the top left. Toggle the Show Custom Header option at the bottom.
• Dismiss the options screen and tap + to show the picker again. Navigate into an album and ensure you see the demo header at the top.
• The demo header should have a green background, which is being overridden by the configureCustomHeaderView delegate method (the demo header class has a red background on initialization).

[frosty]

Thanks for the comment @SergioEstevao! So in the case where a camera capture header and a custom header are enabled, only the custom header will display.

In my upcoming PR for WordPress-iOS, I've only added the permissions alert to screens that don't currently use the capture cell – Media Library +, Gutenberg, Stories. In the other areas, we could just add "Take a photo" to the action sheet that appears before displaying the media picker and not use the capture cell there any more. This may actually be clearer for the user and is would be more consistent across the app.