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/input/doc.go

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+/*
+Package input implements input routing and tracking of interface
+state for a window.
+
+The [Source] is the interface between the window and the widgets
+of a user interface and is exposed by [gioui.org/app.FrameEvent]
+received from windows.
+
+The [Router] is used by [gioui.org/app.Window] to track window state and route
+events from the platform to event handlers. It is otherwise only
+useful for using Gio with external window implementations.
+*/
+package input