please add gconf key support to set the --minport argument

Forwarded from
https://launchpad.net/distros/ubuntu/+source/gnome-btdownload/+bug/5374

many people, myself included, are in the most
unfortunate situation of having only say port 6882
open, the default behaviour of gnome-btdownload would
make filetransfer on such clients less effective.

The problem quite specifically is that bitorrent uses
the 6991 to 6889 port range, and my setup only allows
6882 alotted for my client machine, however bittorrent
will start out by trying 6881, giving worse transfer
speed. However the client accepts a commandline
argument called --minport to get around this issue, I'm
suggesting that since this is a little used but very
useful setting it might be worth allowing to set it via
gconf so to not clutter hte interface.