Merge #15136: qt: "Peers" tab overhaul

3537c83 Do not deselect peer when switching away from tab (Hennadii Stepanov)
b0037c5 Improve Peers tab layout (Hennadii Stepanov)

Pull request description:

  This is an alternative to #14798.

  The "Peers" tab of the "Debug" window improved to address comments bitcoin/bitcoin#6209 (comment) (by @jonasschnelli) and bitcoin/bitcoin#14798 (comment) (by @promag).

  This allows to keep the peer selection while navigating to other places and effectively reverts e059726.

  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)

Tree-SHA512: 3d086007f6d72930bc2fc3c395175adda0f1a7722de3842bc246ee4f3bfc5ebda4b9a626fb68a7ee8663a88d0842deb37c0c460ad84cc58e22f138acf8bc71ea

Author: laanwj