Two download manager windows open when private window is active

[LummoxJR]

The ucjsDownloadsManager.uc.js script causes two copies of the download manager to appear when you begin a download, if a private window is open. One copy is for regular downloads, and the other is for private downloads. It's okay that there are two different manager windows, but they should only open when a download is active for that particular window. Instead, downloading in a regular window will cause the private download manager window to appear also.

Steps to reproduce:

1. Open a private window in addition to existing non-private windows.
2. In either the private or regular window, begin a download. (Two download managers will appear. One of them will be empty.)

Expected behavior:

The appropriate download window, normal or private, should only open for whichever download is initiated. A normal and private download manager window should not open at the same time.

This issue is probably related to #72 in which the download manager can't be closed while a download is active. I've reported it separately in case it's a different issue.