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.
Slightly nitpicky, but `qDebug()` is for debug messages, while this should go to `qWarning()`, with a more descriptive prefix than "Error" (something like "Couldn't upload screenshot"). Unless this is temporary until you get a notification API to use, of course, in which case it's fine.