io/key: [API] implement key event propagation

Before this change, every Event would be passed to the focused InputOp
tag, making it impossible to implement, say, program-wide shortcuts.
This change implements key.Event routing similar to how pointer.Events
are routed: every InputOp describes the set of keys it can handle, and
the router use that information to deliver an Event to the matching
handler.

This is an API change, because every InputOp must now include a filter
matching the keys it wants to handle.

Fixes: https://todo.sr.ht/~eliasnaur/gio/395
Signed-off-by: Elias Naur <mail@eliasnaur.com>

io/router/pointer.go

@@ -9,6 +9,7 @@ import (
 	"gioui.org/f32"
 	"gioui.org/internal/ops"
 	"gioui.org/io/event"
+	"gioui.org/io/key"
 	"gioui.org/io/pointer"
 	"gioui.org/io/semantic"
 	"gioui.org/io/transfer"
@@ -40,6 +41,7 @@ type hitNode struct {
 
 	// For handler nodes.
 	tag  event.Tag
+	ktag event.Tag
 	pass bool
 }
 
@@ -258,6 +260,15 @@ func (c *pointerCollector) newHandler(tag event.Tag, events *handlerEvents) *poi
 	return h
 }
 
+func (c *pointerCollector) keyInputOp(op key.InputOp) {
+	areaID := c.currentArea()
+	c.addHitNode(hitNode{
+		area: areaID,
+		ktag: op.Tag,
+		pass: true,
+	})
+}
+
 func (c *pointerCollector) inputOp(op pointer.InputOp, events *handlerEvents) {
 	areaID := c.currentArea()
 	area := &c.q.areas[areaID]
@@ -636,6 +647,19 @@ func (q *pointerQueue) Deliver(areaIdx int, e pointer.Event, events *handlerEven
 	}
 }
 
+// SemanticArea returns the sematic content for area, and its parent area.
+func (q *pointerQueue) SemanticArea(areaIdx int) (semanticContent, int) {
+	for areaIdx != -1 {
+		a := &q.areas[areaIdx]
+		areaIdx = a.parent
+		if !a.semantic.valid {
+			continue
+		}
+		return a.semantic.content, areaIdx
+	}
+	return semanticContent{}, -1
+}
+
 func (q *pointerQueue) Push(e pointer.Event, events *handlerEvents) {
 	if e.Type == pointer.Cancel {
 		q.pointers = q.pointers[:0]