[Direct Transfer] Add a new graphical option to choose the duplicate behavior

You can choose what to do when a transfer would create a duplicate document on the server. The setting is effective for all files that will be sent at the same time (it is called the session ). Each session has its own duplicates behavior. Available options are: Create : a duplicate document will be created, this is the same behavior as when adding a new file from Web UI. Ignore : the transfer will be cancelled, preventing the duplicate creation. Override : the document will be replaced on the server. Note: The Ignore option may be problematic on huge trees: for each and every file, an NXQL query will be done on the server to verify the existence of a similar document. And that query is not using Elasticsearch, so if you have folderish documents with thousands of children, that query will likely be a bottleneck. TODO: screenshot