#2 Replace listboxes with DataGridViews

[felixwatts]

The feed list and item list on the main form are currently ListViews. It might be nice to change these to DataGridViews for a few reasons:

1) Nicer code: we can then use databinding to bind the grid to a list of Feeds/Items and get rid of the nasty 'Feed derives from ListViewItem' situation etc.

2) Should be able to reduce the slight flicker when an item/feed is downloading. This is caused by numerous invalidations of the entire listview in order to update the download percentage of just one item. DataGridView allows you to just invalidate a single item (although may also be able to do this with ListView by invalidating a region).

3) Custom drawing of cells means we could have a funky download progress bar.