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.
I'd prefer if you kept everything settings-related in `implementSettings()` and set a `bool` somewhere. Anything with freetext keys is notoriously error prone, which is why I'd like to keep anything related to them concentrated in as few places as possible.