Qt: Add GUI view of peer information. #4133

Here is my code which adds a "Peers" tab to the debug window.

The peers table lists just a few columns of detail, and then when you click on a peer it shows all of the detail.  It updates every second while the debug window is open (by doing a full scan of vNodes), and also handles the case where the selected peer gets disconnected.

If you have any suggestions on which columns should appear in the table, or how to format some of the values, I'm happy to implement them.  I just chose to show Address, Subversion, and Height in the table for now.  The advantage of including fields in the table is that they are sortable, so I guess the table should contain those properties which users are most likely to want to sort by.

There's a fair bit of code here, so take your time with the review.

Thanks.