A quick addition would be the ability to add elem_filters for type=signal. The filters should support:
- Show/hide meta-signals (signal-everything, signal-anything, signal-each) - need control of each meta-signal independently as sometimes only each or every is appropriate, sometimes every and any are but each isn't, etc.
- Show/hide the "Or set a constant" sub-panel (e.g. from the RHS of the train-stop "Enabled Condition" UI)