Add user defined labels in thread view mode title

Since I identify threads based on the labels in it, this little enhancement allows me to quickly remember what context I'm in. The status line now looks like that:

```
[thread-view-mode] <label1,label2> The subject of this thread    line 1 of 2
```

with label1 and label2 being user-defined labels.

This is only here for discussion. TODO:  update the status line when we modify the tags inside the `thread-view-mode`