Label navigation

[zspitz]

Label navigation has two parts:

1. from a LabelExpression, navigate to one or more GotoExpressions
2. from a GotoExpression, navigate to the target LabelExpression

The UI for this could be in the context menu.

Alternatively, we could put it inline in the tree node:

1. an arrow with a dropdown, for one or more GotoExpressions (we might make the dropdown conditional on being more than one GotoExpressions)
2. an arrow for navigating to the target LabelExpression