Fix touch detection for QtWebKit

See similar changes made to https://github.com/johndyer/mediaelement/pull/939 and other open source JS projects.

According to the discussion in the linked PR, QtWebKit's behavior here is in line with the specification and not a bug.

Doing some other tests:
-Chrome 28 on a Windows Surface has window.ontouchstart == null
-Firefox 20 on a Windows Surface has window.ontouchstart == null
-IE 10 on a Windows Surface doesn't define window.ontouchstart