Feature Request - Warning Manager - Add timers as triggers

#### Issue details

In the warning manager we can assign trigger values for variables that are greater than x, it would be handy to be able to set custom triggers based on variables based on time. For instance Ch6out is held high for 5 minutes = trigger alarm.

Start by locating the Warning Manager and its existing variable trigger evaluation for values greater than a threshold. Trace how trigger state is tracked, then define how a time-based condition such as Ch6out remaining high for five minutes should be configured and detected; done means that condition can raise an alarm reliably.

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