xdg-toplevel states should define a starting value for externally defined states

I believe the intention in xdg-toplevel was to allow other protocols to define custom states, but there is no guidance on what enum values should be allowed for this.

The xdg-shell protocol (and other protocols with states in an array) should define a starting index for custom states. Some starting value like 65536 should give plenty of room for future, official xdg-shell states while giving plenty of room for custom states defined in other protocols.