CHANGES:

• Add Lwd.update (by @OlivierNicole and @Julow) and Lwd.may_update functions
• Lwd.set: change binding before invalidation, otherwise the old value could be re-observed (reported by @voodoos)
• brr-lwd: support declarative events and set of css classes
• Nottui: fix treatment of some terminal events that were delayed because of improper buffer flushing, reported by @darrenldl
• Introduce Nottui_unix: move platform-specific code of Nottui to a separate library (by @dinosaure and @reynir, #44, #51)