Allow open to be asynchronous

This stops the ui being blocked while the resource is opened. The wait
ensures that resources are reclaimed when the process finishes while
aerc is still running.

commands/msgview/open.go

@@ -60,10 +60,9 @@ func (Open) Execute(aerc *widgets.Aerc, args []string) error {
 			return
 		}
 
-		err = lib.OpenFile(tmpFile.Name())
-		if err != nil {
+		lib.OpenFile(tmpFile.Name(), func(err error) {
 			aerc.PushError(" " + err.Error())
-		}
+		})
 
 		aerc.PushStatus("Opened", 10*time.Second)
 	})