Select user's preferred mimetype in MessageViewer

This implements selecting the most preferred mimetype under the
'View->Alternatives' configuration setting when viewing a message.
Mimetypes in the alternatives array are weighted by their position,
where the lower the index in the array the higher the priority, so this
is taken into account during selection.

If no message part matches a mimetype in the alternatives array, then it
selects the first mimetype in the message.

widgets/msgviewer.go

@@ -160,14 +160,25 @@ func createSwitcher(switcher *PartSwitcher, conf *config.AercConfig,
 		if err != nil {
 			return err
 		}
-		switcher.selected = -1
+		selectedPriority := -1
 		for i, pv := range switcher.parts {
 			pv.OnInvalidate(func(_ ui.Drawable) {
 				switcher.Invalidate()
 			})
-			// TODO: switch to user's preferred mimetype, if configured
+			// Switch to user's preferred mimetype
 			if switcher.selected == -1 && pv.part.MIMEType != "multipart" {
 				switcher.selected = i
+			} else if selectedPriority == -1 {
+				for idx, m := range conf.Viewer.Alternatives {
+					if m != pv.part.MIMEType+"/"+pv.part.MIMESubType {
+						continue
+					}
+					priority := len(conf.Viewer.Alternatives) - idx
+					if priority > selectedPriority {
+						selectedPriority = priority
+						switcher.selected = i
+					}
+				}
 			}
 		}
 	}