[backport#16090] Qt: Add vertical spacer to peer detail widget

Summary:
36b0a2f2a6b49008c4f37866a9e3ab702eb34eda Add vertical spacer (Josu Goñi)

Pull request description:

Before:

![image](https://user-images.githubusercontent.com/25986871/58375408-a8f22c80-7f52-11e9-96ca-14f2186e6fa7.png)

After:

![image](https://user-images.githubusercontent.com/25986871/58375420-fa022080-7f52-11e9-8add-eafe98068e8d.png)

Backport of Core PR16090

Test Plan:

ninja all
bitcoin-qt -testnet

Check out how nicer it looks

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8664