Add screenshoter

I think it's would be useful.
It's capture image from the webView widget, uploads it on imgur and copy to clipboard direct link to uploaded file. I checked it on my Arch linux and it builds and working fine.
My C++ and Qt knowledge is very low so code may (should) contain some mistakes. Any comment and fixes are welcome.
Also, a bit nitpicky, but screenshotBox is not the best name you could have picked for that.
`uploadScreenshotsCheckbox` is more consistent. (I don't know why `proxyPortBox` is named like that though...)