Implement event filtering

[jdalbey]

In the "Simulation Script" tab, implement the event filter for all event types.
E.g., selecting "CHP CAD" from the combo box will display only CHP CAD events and hide all others.

For Telephone Conversation event types, it would be nice to further select by role, e.g., "TV Reporter #1". I don't know what the UI would look like for this; a popup selection? a secondary combo box? For now, let's just worry about the primary filter. This secondary filter can be worked around using the browser search feature (Ctrl-F) to search for specific roles.