When we see a visitor with a correctly formatted &gclid= parameter, in the landing page url, then we can set this visitor as coming from adwords.
gclid should be at least 10 characters long, and less than 100.
Ideally we would also store the gclid value in the log_visit table for later use, eg. importing analytics conversions from Piwik into Adwords.
This would likely be done in a new plugin
Follows up #4261