This is an alternative to #14798.

The "Peers" tab of the "Debug" window improved to address comments https://github.com/bitcoin/bitcoin/pull/6209#issuecomment-108072605 (by @jonasschnelli) and https://github.com/bitcoin/bitcoin/pull/14798#issuecomment-441618268 (by @promag).

This allows to keep the peer selection while navigating to other places and effectively reverts e0597268116cf90d961abeba9d14aaad0ab682d2.

Screenshots with this PR:
![screenshot from 2019-01-09 22-01-36](https://user-images.githubusercontent.com/32963518/50927352-2e6fb700-1460-11e9-9173-582348210492.png)
![screenshot from 2019-01-09 22-02-11](https://user-images.githubusercontent.com/32963518/50927354-329bd480-1460-11e9-9926-d0eb0f026a35.png)
![screenshot from 2019-01-09 22-02-37](https://user-images.githubusercontent.com/32963518/50927358-3596c500-1460-11e9-864d-c8704451f3d9.png)

ninja all
./qt/bitcoin-qt -testnet