Provide support for context menus

[sjchmiela]

Instagram#1430 +:

• merged latest master in
  • the only conflict was in CHANGELOG
• added return to collectionView:contextMenuConfigurationForItemAt:point: as per this comment
• fixed indentation in existing changes to the codebase
  • thanks for letting me know of Reindent!
• removed context menu methods from selection delegate as per this comment
• added support for preview methods too (we need it to customize UITargetedPreview)
  • I left/added support for implementing context menu in section controllers even though we are not using it right now. I think author of the original pull request intended to let developers implement context menu methods in either collectionViewDelegate of the IGListAdapter or section controllers (see this comment).
  • Since we do receive indexPath when asked about context menu configuration it's easy to grab the appropriate section controller.
  • It is not so easy when we're asked for the targeted preview though where we only receive the configuration.
    • In ChatViewController I used configuration.identifier = indexPath as NSCopying to be able to figure out for which message the preview is. Here we can't really do that — it's the delegate who should be able to set their own identifier, we can't override it in library.
    • To match configuration and section controller I added an NSMapTable with weak keys and weak values — as long as configuration and section controller are used/remembered/retained by other objects we will remember them in the table. As long as they are to be forgotten – they are since the references we hold are weak.

[amalenduk]

Hi @sjchmiela, The example you have added doesn't work at all, I see you are returning nil (UIContextMenuConfiguration *)collectionView:(UICollectionView *)collectionView contextMenuConfigurationForItemAtIndexPath:(NSIndexPath *)indexPath point:(CGPoint)point in this function you should have returned configuration otherwise it doesn't work at all