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/example_test.go

@@ -110,7 +110,7 @@ func ExampleDraggable_Layout() {
 
 		// Check for the received data.
 		for {
-			ev, ok := gtx.Event(&drop, transfer.TargetFilter{Type: mime})
+			ev, ok := gtx.Event(transfer.TargetFilter{Target: &drop, Type: mime})
 			if !ok {
 				break
 			}