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/dnd.go

@@ -69,7 +69,7 @@ func (d *Draggable) Update(gtx layout.Context) (mime string, requested bool) {
 	d.pos = pos
 
 	for {
-		e, ok := gtx.Event(d, transfer.SourceFilter{Type: d.Type})
+		e, ok := gtx.Event(transfer.SourceFilter{Target: d, Type: d.Type})
 		if !ok {
 			break
 		}