Parse headers from template

Parse the processed template for headers and populates matching header
editors accordingly.
Those are then stripped from the template body before prepending it
and remaining header fields to the composer content.

The motivation for this is keeping receiver, sender and subject
lines in the template file and generating the message subject with the
date functions.

widgets/compose.go

@@ -179,8 +179,7 @@ func (c *Composer) AddTemplate(template string, data interface{}) error {
 	if err != nil {
 		return err
 	}
-	c.PrependContents(bytes.NewReader(templateText))
-	return nil
+	return c.addTemplate(bytes.NewReader(templateText))
 }
 
 func (c *Composer) AddTemplateFromString(template string, data interface{}) error {
@@ -192,7 +191,44 @@ func (c *Composer) AddTemplateFromString(template string, data interface{}) erro
 	if err != nil {
 		return err
 	}
-	c.PrependContents(bytes.NewReader(templateText))
+	return c.addTemplate(bytes.NewReader(templateText))
+}
+
+func (c *Composer) addTemplate(tmpl io.Reader) error {
+	reader, err := mail.CreateReader(tmpl)
+	if err != nil {
+		return errors.Wrap(err, "mail.CreateReader")
+	}
+	defer reader.Close()
+
+	// populate header editors
+	header := reader.Header
+	mhdr := (*message.Header)(&header.Header)
+	for _, editor := range c.editors {
+		if mhdr.Has(editor.name) {
+			editor.input.Set(mhdr.Get(editor.name))
+			mhdr.Del(editor.name)
+		}
+	}
+
+	part, err := reader.NextPart()
+	if err != nil {
+		return errors.Wrap(err, "reader.NextPart")
+	}
+	c.PrependContents(part.Body)
+
+	var (
+		headers string
+		fds     = mhdr.Fields()
+	)
+	for fds.Next() {
+		headers += fmt.Sprintf("%s: %s\n", fds.Key(), fds.Value())
+	}
+	if headers != "" {
+		headers += "\n"
+	}
+
+	c.PrependContents(bytes.NewReader([]byte(headers)))
 	return nil
 }