otioview's CompositionWidget assumes all stacks have tracks

otioview CompositionWidget can run into issues when coming across nested stacks that contain direct clip (Item) children. You'll notice this if you have a:

Timeline -> Stack -> Track -> Stack -> Clip

And you try and "Open" (double click for example) the internal Stack you'll see:
```
File "...OpenTimelineIO/opentimelineview/timeline_widget.py", line 375, in _adjust_scene_size
for t in self.composition
File "...OpenTimelineIO/opentimelineview/timeline_widget.py", line 375, in
for t in self.composition
AttributeError: 'Clip' object has no attribute 'kind'
```
This assumption shows up in both `_adjust_scene_size()` and `_add_tracks()`

Start in opentimelineview/timeline_widget.py, focusing on _adjust_scene_size() and _add_tracks(), and reproduce the nested Timeline → Stack → Track → Stack → Clip case by opening the internal Stack. Done means CompositionWidget handles direct Clip children without the reported AttributeError in either function.

Written by the indexing model from the issue text.