io/input: [API] introduce Source, the interface between a Router and widgets

This change gets rid of the event.Queue interface by replacing it with
input.Source values. Source provides the interface to Router necessary
to implement interface widgets.

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

io/event/event.go

@@ -1,42 +1,8 @@
 // SPDX-License-Identifier: Unlicense OR MIT
 
-/*
-Package event contains the types for event handling.
-
-The Queue interface is the protocol for receiving external events.
-
-For example:
-
-	var queue event.Queue = ...
-
-	for _, e := range queue.Events(h) {
-		switch e.(type) {
-			...
-		}
-	}
-
-In general, handlers must be declared before events become
-available. Other packages such as pointer and key provide
-the means for declaring handlers for specific event types.
-
-The following example declares a handler ready for key input:
-
-	import gioui.org/io/key
-
-	ops := new(op.Ops)
-	var h *Handler = ...
-	key.InputOp{Tag: h, Filter: ...}.Add(ops)
-*/
+// Package event contains types for event handling.
 package event
 
-// Queue maps an event handler key to the events
-// available to the handler.
-type Queue interface {
-	// Events returns the available events for an
-	// event handler tag.
-	Events(t Tag) []Event
-}
-
 // Tag is the stable identifier for an event handler.
 // For a handler h, the tag is typically &h.
 type Tag interface{}