Relates to #7734

![image](https://user-images.githubusercontent.com/226170/33406640-8ea700c6-d576-11e7-9d69-fde9a696c219.png)

Please ignore the wrong alpha in the screenshot, I couldn't get the screenshot alpha right :(

I plan to extend this feature in future PRs to include:

- custom Tor icon
- clickable icon which opens network settings

Old proposals, dropped in favor of current

![image](https://user-images.githubusercontent.com/226170/32688635-979ef690-c6dd-11e7-8869-49da7e0f0a11.png)
![proxy_preview](https://user-images.githubusercontent.com/226170/31521305-99c43f22-afb1-11e7-9daf-d1ed6347daa8.png)
![image](https://user-images.githubusercontent.com/226170/31680585-72706098-b37d-11e7-88ad-028c4c723f42.png)

make check
bitcoin-qt -proxy=<valid proxy>

bitcoin-qt