Improve ProgressBar appearance in the "% Complete" column

I noticed that the "% Complete" column of the torrent list doesn't look quite ideal, and is also hard to read sometimes, at least in the selected and highlighted rows. These patches aim to fix that.

Here are a few "before" screenshots, with my default style, and with "Fusion", "Adwaita", and "Adwaita-Dark":

The same content and styles, "after":

I admit that the second commit is the result of some guesswork and experimentation, and the first one might not be the most elegant either, but the table looks to me much better than before.

I wasn't able to test these changes on other platforms, only on Fedora KDE.