Handle for gridOverlayView

[UR27]

The behavior of the gridOverlayView when cropping is slightly different depending on whether it is dragged by holding a corner or by holding an edge.

It would be easier for users to understand the difference in this behavior if there were handles in the center of the gridOverlayView's edges, as in Apple's Photos app for crop editing.

CropViewController is actually used in many of my apps.
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