"Install .plt File" dialog allows and hides multiline input

Today I copied this URL to my clipboard, including a trailing newline: http://www.cs.indiana.edu/classes/c211/c211-handin.plt Then I chose "File" -> "Install .plt File" and pasted into the "Web" tab. The "URL:" text box remained empty, which was confusing. I tried pasting a few more times before giving up and typing the URL by hand. I got the following mysterious error, which has been reported by my students before:

There was an error when checking the validity of the downloaded .plt file. Please check the url and try again.

not an unpackable distribution archive

gnu-unzip: bad header

By hitting cursor keys while focused on the "URL:" text box, I discovered that the problem is the text box allows multiple lines but is only tall enough to show one line. I think the right behavior is to allow only one line, trim surrounding whitespace, and turn internal whitespace into space. The same issue seems to afflict the "File" tab as well.

Start by reproducing the issue in the "Install .plt File" dialog, checking the URL text box in both the Web and File tabs with multiline and surrounding-whitespace input. Done means each tab accepts the input as a single-line value, trims surrounding whitespace, converts internal whitespace to spaces, and no longer hides pasted content.