Fix item label for constraint to show their name and expression

[pcdavid]

Currently in some contexts (e.g. in the require constraints compartment of a _requirement def), an item for a constrinat which has an expression shows the expression, but not the name of the constraint:

In other contexts, e.g. in the constraints compartment of a constraint, the item shows the name of the constraint but not its value:

In all cases where a constraint element is shown as a list items, the item's text should be consistent and show:

• the name if set
• the expression between {} if set.

For the examples in the screenshots above, we should have:

• samplingInBounds { s.samplingRate >= 1.0 & s.samplingRate <= 10000.0 }
• contraint1 { true }