Allow timers to trigger timers (#8043)

Unlike any other unit type, it makes sense for a timer to start another
timer. It is an easy way to crate logical "and" between time conditions

for instance, every day but no less than 5' after boot can easily be
implemented by a OnBootSec triggering an OnCalendar.

This is particulary usefull with Persistant timers which tend to all fire
together at startup