Publish "on_current_layer_changed_event" of LayoutView

[klayoutmatthias]

This event is available in C++, but not in Python/Ruby

[klayoutmatthias]

The new event is "on_current_layer_changed". Note that is has an argument: the iterator pointing to the new current layer.

I added one more event: "on_selected_layers_changed" is called when the layer selection has changed - for example after "Select All". This does not issue a "on_current_layer_changed" event.