Add Qt clipboard integration

The GTK clipboard integration isn't working for us and we need a way
to get clipboard requests from the renderer into the system clipboard.

This patch adds a Qt implementation to let Qt handle the platform
abstraction, even though the Chromium clipboard direct integration
could work on some platforms.

Change-Id: Ie7af5c6b4c662fc8f4109b2e85e9f594d6d90b0d
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>