A GLMPresentation does not allow to get the selected value if the option noSelection
is specified. It looks right but ... noSelection is used if we don't want to trigger
a transmission to the selection port (and open a new pane in GTInspector for example).
In this case, it makes sense to always populate the rawSelection so that the selection
remains available to other actions.

To do that, only a small change is needed in GLMPresentation>>selection:
selection: anObject
    "Tell the pane about the new selection. This method is called when the user changes
a view's selection' (e.g., he clicks on an element in a list morph)." 
    (self pane port: #rawSelection) value: anObject.
    self acceptsSelection ifFalse: [^ self].
    rawSelectionTransmissions do: #transmit