iOS 11 drag and drop support

[overheadhunter]

Add support for drag and drop.

When dropping a file into an unlocked vault, it should be added to the current folder.

When dragging a file from an unlocked vault, it should be made available for dropping it onto third party apps.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.