io/system: add ActionInputOp to register window move gesture areas

The app.Window.Perform(ActionMove) is the wrong abstraction for
initiating a move gesture: Windows needs to know the move gesture
area at pointer move, and macOS needs to know the pointer button
down event that triggers the move gesture. This change replaces
Perform(ActionMove) with a new system.ActionInputOp that marks an
area movable.

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

io/router/pointer.go

@@ -13,6 +13,7 @@ import (
 	"gioui.org/io/key"
 	"gioui.org/io/pointer"
 	"gioui.org/io/semantic"
+	"gioui.org/io/system"
 	"gioui.org/io/transfer"
 )
 
@@ -97,6 +98,7 @@ type areaNode struct {
 		id      SemanticID
 		content semanticContent
 	}
+	action system.Action
 }
 
 type areaKind uint8
@@ -256,6 +258,12 @@ func (c *pointerCollector) keyInputOp(op key.InputOp) {
 	})
 }
 
+func (c *pointerCollector) actionInputOp(act system.Action) {
+	areaID := c.currentArea()
+	area := &c.q.areas[areaID]
+	area.action = act
+}
+
 func (c *pointerCollector) inputOp(op pointer.InputOp, events *handlerEvents) {
 	areaID := c.currentArea()
 	area := &c.q.areas[areaID]
@@ -424,6 +432,19 @@ func (q *pointerQueue) semanticIDFor(content semanticContent) SemanticID {
 	return id.id
 }
 
+func (q *pointerQueue) ActionAt(pos f32.Point) (system.Action, bool) {
+	for i := len(q.hitTree) - 1; i >= 0; i-- {
+		n := &q.hitTree[i]
+		hit, _ := q.hit(n.area, pos)
+		if !hit {
+			continue
+		}
+		area := q.areas[n.area]
+		return area.action, area.action != 0
+	}
+	return 0, false
+}
+
 func (q *pointerQueue) SemanticAt(pos f32.Point) (SemanticID, bool) {
 	q.assignSemIDs()
 	for i := len(q.hitTree) - 1; i >= 0; i-- {