Fixes #649.

I manually tested the following actions to confirm the expected behavior:

1. With a file that belongs to a package open in the current editor tab, selecting Application > Download app downloads the parent package, named according to the package's metadata defined in its expath-pkg.xml file: @abbrev + - + @version + the .xar filename extension.
2. Selecting File > Download downloads the file open in the current editor tab.
3. In File > Manage, selecting one or more resources and/or one or more collections causes the resource(s) to be downloaded as is and the collection(s) to be downloaded zipped.

I lack the skills to craft an automated integration test.