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.
Do it the other way around, make a member in `KVScreenshooter` and set that from `implementSettings()`. Singletons should be independent in as much as possible.