Support for Drag and Drop

[aaronsky]

This pull request is a Work In Progress and should not be merged until otherwise noted!!

This PR introduces drag and drop support to BrickKit.

Bricks that should be considered eligible for drag and/or drop should provide a delegate for Brick#dragDropDelegate. To opt-in to drag, return a [UIDragItem] in BrickDragDropDelegate#dragItems(for:at:). To opt-in to drop, return a Bool in BrickDragDropDelegate#canHandle(_:) affirming whether the provided UIDropSession has any objects that conform to your brick's supported UTIs, and implement the BrickDragDropDelegate#willDrop(_) and BrickDragDropDelegate#drop(_:to:from:) handlers. For more information, see the DragDropBrickViewController example, as well as Apple's documentation on Drag and Drop in Collection Views.

I have put this PR up here despite its state of completion to kickstart discussion about this feature, as well as provide advice on how BrickLayouts work.

Current list of to-dos:

• [ ] Fix current blocking crashers
  • [ ] Sample app crashes while attempting to reorder layout while previous drop has not completed
  • [ ] Sample app crashes while attempting to reorder layout to accommodate new cells
• [ ] Document all new methods and protocols
• [ ] Implement drag and drop in other example controllers to demonstrate drag and drop between collection views within the same app
• [ ] Write lots of unit tests

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@f3cb712). Click here to learn what that means. The diff coverage is 2.3%.

@@           Coverage Diff            @@
##             master    #190   +/-   ##
========================================
  Coverage          ?   89.6%           
========================================
  Files             ?      41           
  Lines             ?    4626           
  Branches          ?     362           
========================================
  Hits              ?    4145           
  Misses            ?     480           
  Partials          ?       1

Impacted Files	Coverage Δ
Source/Cells/BrickCell.swift	98.67% <ø> (ø)
Source/Layout/BrickZIndexer.swift	100% <ø> (ø)
Source/Layout/BrickFlowLayout.swift	91.18% <ø> (ø)
Source/Cells/ImageDownloader.swift	79.31% <ø> (ø)
Source/Models/BrickSection.swift	85.86% <0%> (ø)
Source/Cells/BrickPlaceholderCell.swift	0% <0%> (ø)
Source/Models/Brick.swift	71.23% <0%> (ø)
...ViewControllers/BrickViewController+DragDrop.swift	0% <0%> (ø)
Source/ViewControllers/BrickViewController.swift	86.11% <100%> (ø)

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[aaronsky]

Before this PR can be merged in, I suspect that BrickKit is going to need to drop support for building against the iOS 10.3 SDK and lower. Otherwise, Travis builds will continue to fail and codecov won't be able to get usable data to work with. This is not the same as dropping support for iOS 9 and 10, we just need to make sure we're only building against the iOS 11.0 SDK or later.