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.
Since `KVScreenshooter` is a singleton now, it'd make more sense to keep this in the singleton itself. That way, you could just call `KVScreenshooter::sharedInstance().screenshot(widget)` anywhere, like this its use is limited to within `KVMainWindow`.