ui: merge package input

Event handling is as fundamental as operations, so move the input
package declarations to package ui.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/input.go

@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package ui
+
+// Queue maps an event handler key to the events
+// available to the handler.
+type Queue interface {
+	// Next returns the next available event, or
+	// false if none are available.
+	Next(k Key) (Event, bool)
+}
+
+// Key is the stable identifier for an event handler.
+// For a handler h, the key is typically &h.
+type Key interface{}
+
+// Event is the marker interface for events.
+type Event interface {
+	ImplementsEvent()
+}