Connect: Improve Aspera Connect folder creation feature

Proposal: Aspera Connect downloads should always obey the transfer spec `create_dir` and `destination_root` properties and behave consistently, whether the user relies on a pre-set base download directory or interactively chooses a base download directory when "Always ask me where to save downloaded files" is enabled.

The current behavior for a download with a transfer spec that includes `{"create_dir": true, "destination_root": "download-1"}` is very different depending on whether the user has configured Aspera Connect to always use a certain base download directory or has enabled the "Always ask me where to save downloaded files" setting.

If that setting is not enabled, and the download directory is configured as `/downloads`, then two files named `file1.txt` and `file2.txt` with a transfer spec that includes those settings will result in the following structure after the download completes:

/downloads/download-1/file1.txt

/downloads/download-1/file2.txt

If the "Always ask me where to save downloaded files" is enabled, and the user chooses the `/downloads` directory, then for the same transfer spec, the `destination_root` directory gets created but the files don't get downloaded into that directory, so it remains empty and the following structure is created:

/downloads/download-1

/downloads/file1.txt

/downloads/file2.txt

The proposal is for Aspera Connect to behave the same in these two cases, so that regardless of whether the user chooses a base download directory each time or always uses the same base download directory (whether the default or not), Aspera Connect behaves consistently, with the following result:

/downloads/download-1/file1.txt

/downloads/download-1/file2.txt

The API documentation currently implies that it works the way I'm proposing it should work, and there is no mention of the very different behavior in the two cases, but Aspera Engineering is adamant that it's supposed to work as it does currently.

The reason it's important to support this is that it's very common to want all files of a single download to be downloaded to a custom subdirectory with a name provided by the server that is not known by the end user, and this is only possible currently if the user does not use the "Always ask me where to save downloaded files".