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.
`static` should go only on the declaration of `sharedInstance`, after that you call `KVScreenshooter::sharedInstance().whatever()`. Note that it's a reference, not a pointer, so you use `.` instead of `->` to access things inside it. (Use `&KVScreenshooter::sharedInstance()` if you for some reason need a pointer, not that you should need to do that.)