#40 Show variable value in VariableBox

[Reuben Grinberg]

Currently, the current value of a variable is not displayed for a field in
the VariableBox. To display the field, we need simly to uncomment
the following line in VariablePanel's constructor:
//FieldTableDescriptor.VALUE_TITLE,

The thing is, it'd be confusing to have different VariableBoxes for
different actual objects. So we want a label
"Value in "

and then a JComboBox of the available objects. Thankfully, we
already have a ListModel (the DObjectManager) that we just plop into
a JComboBox for this purpose.