sortingDelegate additions: allow/disallow movement and insertion of items at certain index

[timtrautmann]

Extended the sortingDelegate with two more optional delegate methods:

- (BOOL)GMGridView:(GMGridView *)gridView shouldAllowMovingCell:(GMGridViewCell *)view atIndex:(NSInteger)index; is called when a move is about to start. If this method returns NO, the move won't take place.

- (BOOL)GMGridView:(GMGridView *)gridView shouldAllowMovingCell:(GMGridViewCell *)view toIndex:(NSInteger)index; is called when an item is moved to a new position in the grid. If this method returns NO, the insertion won't take place.

[gmoledina]

Thanks @Bringo. I'll review this with the new code base and merge.

[ghrom]

Will it be merged, please?

[chadpod]

Also wondering if this will be merged soon. Thanks.

[robertomg]

This feature is very interesting, when will it be merged?

[dvkch]

Still no news on when it'll be merged?

[kushal297]

ummmmmmm why not keep a simple bool for allowing dragging of cell and based on that flag add or remove the longtapgesture