msgviewer: Add Labels as a virtual header

In order to accommodate for that, the headerlayout needed to be rewritten
to pass the filter criteria back to the msgviewer, instead of just using
the normal headers.

widgets/msgviewer.go

@@ -52,7 +52,16 @@ type PartSwitcher struct {
 func NewMessageViewer(acct *AccountView, conf *config.AercConfig,
 	store *lib.MessageStore, msg *models.MessageInfo) *MessageViewer {
 
-	layout := HeaderLayout(conf.Viewer.HeaderLayout).forMessage(msg)
+	hf := HeaderLayoutFilter{
+		layout: HeaderLayout(conf.Viewer.HeaderLayout),
+		keep: func(msg *models.MessageInfo, header string) bool {
+			if fmtHeader(msg, header, "2") != "" {
+				return true
+			}
+			return false
+		},
+	}
+	layout := hf.forMessage(msg)
 	header, headerHeight := layout.grid(
 		func(header string) ui.Drawable {
 			return &HeaderView{
@@ -109,6 +118,8 @@ func fmtHeader(msg *models.MessageInfo, header string, timefmt string) string {
 		return msg.Envelope.Date.Local().Format(timefmt)
 	case "Subject":
 		return msg.Envelope.Subject
+	case "Labels":
+		return strings.Join(msg.Labels, ", ")
 	default:
 		return msg.RFC822Headers.Get(header)
 	}
@@ -580,6 +591,11 @@ func (pv *PartViewer) attemptCopy() {
 						"%s: %s\n", fields.Key(), fields.Value())
 					pv.sink.Write([]byte(field))
 				}
+				// virtual header
+				if len(pv.msg.Labels) != 0 {
+					labels := fmtHeader(pv.msg, "Labels", "")
+					pv.sink.Write([]byte(fmt.Sprintf("Labels: %s\n", labels)))
+				}
 				pv.sink.Write([]byte{'\n'})
 			}