Active logout does not see CMNG Ajax actions

The active logout feature of the CMS logs out a user after X minutes of inactivity. 'Activity' is counted as 'Ajax calls' to the backend. However, none of the Ajax calls done by the next-generation channel manager is counted as user activity. As a result, a user that only uses the channel manager for more than half an hour (by default) will be logged out of the CMS without warning because the system assumes he has been idle during that time.

Steps to reproduce:
1. start the CMS in production mode (e.g. NOT in Wicket Development mode, because the active logout is disabled in development mode).
2. in the console, set /hippo:configuration/hippo:frontend/cms/cms-
static/root/max.inactive.interval.minutes to '1'
3. login to the CMS and open a channel
4. spent a minute doing only channel manager actions (do some edit actions in the channel, toggle the sidenav a bit, edit the menu, etc.)

Expected: after a minute you stay logged in to the CMS
Actual: after a minute you'll be logged out automatically