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 functions aren't actually member functions at all; they don't operate on an object, which is why using `this` inside it doesn't make sense. This is also why it should be used only when it makes sense, like for functions that return a shared instance - such as `KVScreenshooter::sharedInstance()` - and nowhere else.