Pre-scan destination and provide consolidated file replace dialog at start

Please take a look on how NiceCopier (http://sourceforge.net/projects/nicecopier/) pre-scans the destination and provides an ability to choose an action for all or individual files that are conflicting; now I don't really like the UI of NiceCopier which is why I use CopyHandler.

The idea is that we avoid prompts halfway through a few hour long copy/move operation.

See also: https://windows.uservoice.com/forums/265757-windows-feature-suggestions/suggestions/6991101-windows-copy-management