ui: add package input for merged input

To avoid passing a queue type for each kind of input (pointer, key),
introduce package input for mapping a handler key to all input events.

Future input sources can be added without changes to programs, and
as an added bonus, event ordering is preserved across input sources.

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

ui/input/input.go

@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+// Package input exposes a unified interface to input sources. Subpackages
+// such as pointer and key provide the interfaces for specific input types.
+package input
+
+// Events maps an event handler key to the events
+// available to the handler.
+type Events interface {
+	For(k Key) []Event
+}
+
+// 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 input events.
+type Event interface {
+	ImplementsInputEvent()
+}