add possibility to react on will display header and footer

[rad3ks]

As reported in #78 I decided to make small PR with possibility to quickly access two methods of UITableViewDelegate:

• tableView(_:, willDisplayHeaderView:, forSection:)
• tableView(_:, willDisplayFooterView:, forSection:)

The reason we need to have the simplest possibility to access those method is that UIKit somehow overrides header view appearance (for UITableViewHeaderFooterVIew) and those two methods can be used for configuring header/footer correctly.

The solution here is to provide two closures:

• dataSource.willDisplayHeader: ((UIView, Int) -> Void)?
• dataSource.willDisplayFooter: ((UIView, Int) -> Void)?

I wish we could have better architecture for that than just a properties with closures, but this solution fulfills two most important requirements:

• provides expected feature
• don't introduce any breaking change into Static

[dmiluski]

Coming in way late to this PR, but what is it's use here? Tracking? Animations? Resizing?

[txaiwieser]

I would really like to access scrollViewDidScroll: its not possible to make this generic?

[dmiluski]

@txaiwieser access scrollViewDidScroll: ? Oftentimes, if there are several parties involved, I steer clear of delegate methods unless I for sure own that delegate.

In your case, could you leverage an observer:

// Add an observer (be sure to remove in deinit or viewWillDisappear based on your use case)
tableView.addObserver(self, forKeyPath: "contentOffset", options: [NSKeyValueObservingOptions.new, NSKeyValueObservingOptions.old], context: &contentOffsetContext)

    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if context == &contentOffsetContext, let offsetValue = change?[.newKey] as? NSValue {
            let offset = offsetValue.cgPointValue
           // Handle offset Changes as you wish.
        }
    }

[jaredegan]

I believe my PR solves this in the best way: #119

You don't want to need to create a bunch of closures for each message in UIScrollViewDelegate or UITableViewDelegate. And you don't want to need to update them with iOS version updates as new ones get added or existing ones get deprecated.