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/transfer/transfer.go

@@ -42,6 +42,8 @@ func (OfferCmd) ImplementsCommand() {}
 // as well as [InitiateEvent] and [CancelEvent].
 // Use multiple filters to offer multiple types.
 type SourceFilter struct {
+	// Target is a tag included in a previous event.Op.
+	Target event.Tag
 	// Type is the MIME type supported by this source.
 	Type string
 }
@@ -49,6 +51,8 @@ type SourceFilter struct {
 // TargetFilter filters for any [DataEvent] whose type matches a MIME type
 // as well as [CancelEvent]. Use multiple filters to accept multiple types.
 type TargetFilter struct {
+	// Target is a tag included in a previous event.Op.
+	Target event.Tag
 	// Type is the MIME type accepted by this target.
 	Type string
 }