[igx-simple-combox]: Add selectionChanged event

[mikerentmeister]

Is your feature request related to a problem? Please describe.

Yes. IgxSimpleCombo currently emits a selectionChanging event, but there is no way to be notified when the selection has taken effect.

Describe the solution you'd like

Please create a selectionChanged EventEmitter that emits after the new selection has been updated in the selectionService.

Describe alternatives you've considered

I looked at the code for IgxSimpleCombo and the base class, but I didn't see any way to be notified when the selection changes, especially because selection and selectedItems are just getters. The selectionService is protected, and doesn't expose any events either.

In my particular use case I cannot use ngModel to be notified when the value changes (long story).