Right-align file sizes

Currently there's only one numeric column, and that's the file size, so it gets
special treatment.

I was originally going to have a folder file size field be filled up with '-'s
as far as it could go, leaving it entirely up to the column how its field gets
formatted. But then I saw just one '-' working just fine, so I left it like
that. In the first try, columns could do anything they want when padding a
string (including changing the padding character or just changing it entirely),
but now there's no point.
Darken file size suffixes

I think this is necessary because 'bytes' currently has no 'B' suffix, and it's
kind of hard to distinguish a long number from a suffix.