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/router_test.go

@@ -16,7 +16,7 @@ func TestNoFilterAllocs(t *testing.T) {
 		b.ReportAllocs()
 		b.ResetTimer()
 		for i := 0; i < b.N; i++ {
-			s.Event(nil, pointer.Filter{})
+			s.Event(pointer.Filter{})
 		}
 	})
 	if allocs := b.AllocsPerOp(); allocs != 0 {