Message dialog is cramped when message is shorter than details

A frequent pattern is something like:
```python
error(
parent=parent,
message="An error occurred!",
detail=traceback.format_exc()
)
```
Because the message is short in length, the `QMessageBox` shrinks to accommodate only that length. But when the user clicks the "Show details" button, the longer message (in this example, the traceback) is incredibly cramped, and there is no method to resize the message box.

[Relevant stackoverflow answer](https://stackoverflow.com/a/57878068/1430829)