Proposal for more options regarding keyboard interactivity for layer-shell clients

Created by: dkondor

I'm opening this as a concrete proposal for #32 (closed)

I believe the current protocol makes it hard to have layer-shell surfaces in the top layer (e.g. panels and docks) that have keyboard interaction on their main surface and not in popups.

Let me know if I should provide more motivation or use cases.

My intention for this is to be backward-compatible with the current protocol (0/1 values for keyboard interactivity keep their meaning).