Allow middle clicking a dashboard top tab to close the dashboard

We allow middle clicking to close a tab within a dashboard, we should also allow middle clicking the dashboard's tab itself to close the whole dashboard.

Example for creating a dashboard:
```python
from deephaven import ui, empty_table

my_dash = ui.dashboard([
  empty_table(10).update("x = i"),
  empty_table(10).update("x = i"),
])
```