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.
Place it on the desktop instead. If screenshots end up in the Pictures folder, it's not immediately obvious where they are, and the user might just think the Screenshot function is broken.