Add tooltip to "Download an OS" rows
Unfortunately, we are still doing a bad job at differentiating the "Download an OS" entries. The reason is the lack of standardization in the media filenames and variant classification.
There's an ongoing discussion in Libosinfo to classify the "purpose" of an Osinfo.Media.
https://www.redhat.com/archives/libosinfo/2018-November/msg00146.html
For now, let's add a tooltip to the rows containing the media filenames.
Fixes #471 (closed)