file-operations: Fix conflict dialog for google-drive

Conflict dialog doesn't work properly for google-drive files, because G_IO_ERROR_EXISTS can be returned even if a destination file based on a basename of a source file doesn't exists. This happens when a destination directory already contains some file with the same display_name as the source file has. Usually, display_name is based on filename, but google-drive is not traditional filesystem and uses unique IDs for files. Although, google-drive supports multiple files with the same display_bane in one folder, the copy and move operations behave traditionally and tries to overwrite existing files with the same title. Let's base the destination file on display_name for google-drive to fix issues with the conflict dialog.

See: gvfs!58 (merged)