Trigger severity instead of problem severity being taken into account when filtering triggers in Monitoring->Overview (Triggers).

Steps to reproduce:

1. Create a trigger with Severity < Disaster.
2. Bring the trigger to the problem state.
3. Update triggered problem by increasing its severity.
4. Navigate to Monitoring->Overview (Triggers) and change Minimum trigger severity to problem severity.

Result:
Trigger is hidden.
Expected:
Trigger must be displayed with severity of its problem.

Most likely filtering triggers by Age less than also is not working properly.