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>

widget/button.go

@@ -148,13 +148,16 @@ func (b *Clickable) Update(gtx layout.Context) (Click, bool) {
 		}
 	}
 	filters := []event.Filter{
-		key.FocusFilter{},
+		key.FocusFilter{Target: b},
 	}
 	if b.focused {
-		filters = append(filters, key.Filter{Name: key.NameReturn}, key.Filter{Name: key.NameSpace})
+		filters = append(filters,
+			key.Filter{Target: b, Name: key.NameReturn},
+			key.Filter{Target: b, Name: key.NameSpace},
+		)
 	}
 	for {
-		e, ok := gtx.Event(b, filters...)
+		e, ok := gtx.Event(filters...)
 		if !ok {
 			break
 		}