Fixed bug: cantouch capability is detected as a false positive in WebKitGTK+

While having a look at https://bugs.webkit.org/show_bug.cgi?id=129680, Diego Pino (dpino@igalia.com) and Enrique Ocaña (eocanha@igalia.com) realized that nicescroll was detecting WebKitGTK+ running on desktop as having "cantouch" capabilities. WebKitGTK+ and, in general, all WebKit based browsers, should have the same treatment as Chrome.