Show conflict dialog before upload when possible

When uploading files, first check if the files exist in the current file
list. For the ones that do, show a conflict dialog.
For the rest, upload directly.

If the upload operation detects a conflict on the server side, it will
also continue populating the conflict dialog.

From now on, server side conflict can only occur if someone concurrently
uploaded a file into the same folder but the current user hasn't
refreshed the list yet.