Add long press preview for images

[smeshko]

Purpose

Adds long press preview to images.

Details

This PR adds support for long press previews of photos/videos.

On iOS versions <13.0 it only shows a preview of the selected asset, while on iOS 13+ it also adds a contextual menu with the option to select/deselect the previewed photo.

The previews support static photos, live photos and videos.

Changes

In order to achieve this, a new read-only property was added to TLPhotoCollectionViewCell:
open internal(set) var asset: PHAsset? as well as 2 delegate callbacks to TLPhotosPickerViewController.

public func previewingContext(_ previewingContext: UIViewControllerPreviewing, viewControllerForLocation location: CGPoint) -> UIViewController? is used for iOS <13 devices and only shows a preview of the photo on a force touch.

public func collectionView(_ collectionView: UICollectionView, contextMenuConfigurationForItemAt indexPath: IndexPath, point: CGPoint) -> UIContextMenuConfiguration? is used for iOS 13+ devices and shows the context menu to select/deselect the photo.

Screenshots

[tilltue]

@smeshko
Thanks for the PR!
I added a preview option, the default option is false to preserve the previous user experience.