all: [API] replace tag parameter of Source.Event with per-filter tags

Until now, every event has had a particular target. We're about to simplify
key event delivery to match the first matching filter, so there is no
longer a global meaning to the tag argument to Source.Event.

Add fields to filters to specify their target tags.

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

io/input/key_test.go

@@ -294,6 +294,7 @@ func TestFocusScroll(t *testing.T) {
 	filters := []event.Filter{
 		key.FocusFilter{},
 		pointer.Filter{
+			Target:       h,
 			Kinds:        pointer.Scroll,
 			ScrollBounds: image.Rect(-100, -100, 100, 100),
 		},
@@ -322,7 +323,8 @@ func TestFocusClick(t *testing.T) {
 	filters := []event.Filter{
 		key.FocusFilter{},
 		pointer.Filter{
-			Kinds: pointer.Press | pointer.Release,
+			Target: h,
+			Kinds:  pointer.Press | pointer.Release,
 		},
 	}
 	assertEventPointerTypeSequence(t, events(r, h, filters...), pointer.Cancel)