Minor updates to comments and tooltips
Add available component names to model labels list
Select model label when driver is selected

When a component is selected as the driver of a model, also select its
label in the list of model labels.
Show labels unchecked when "Open" button selected

The instance of LabelsMenu used with the "Open" button should show all
available labels in an unselected state. The user can then check labels
to filter the list of available models to open.

Note that the selection state of the labels for the current model isn't
modified; the labels are only displayed unchecked in the LabelsMenu
widget.
Deselect model label when driver is deleted

This is the complement to when the driver is added to the model.