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.
Oh, seems I solve it adding `static` to public and protected functions (and pointer). Right?
But now i getting https://gist.github.com/Yukariin/174efdb482967c0b099f