Use an event to collect visitor information and made sure UI works even when minimum plugins are enabled

refs #6045 #6049 

Disabled most or even all tracker and reporting plugins and Piwik UI still works now. There were a couple of issues, for instance the menu, some templates, ... So far there is only one event to add visitor information to the API. I am not sure how to solve the UI part. Would be quite difficult to trigger events there and to make sure everything is in the correct order. So leaving the template part as it is but made sure there won't be any errors in case a property is not set.