figure with subplots: after setting multiple X (or Y) labels, only the one set in first is editable interactively

Reported by Samuel GOUGEON (@sgougeon)

BUG DESCRIPTION:
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plot()
// Right click below the X axis of the bottom axes and choose Label => Label X. Then enter some text and validate: OK
// Double click on the x label you just enter: OK, a edition box opens and we can change the label

// Now, add in the sale way a label to the X axis of the upper axes: OK, it works
// Now, double-click on it => KO: nothing happens : it is not possible to edit it

// Now double-click on the X label of the bottom axes: OK, it still work
// So, clear it and validate
// Now double-click on the X label of the UPPER axes: now IT WORKS

The same trouble occurs for the Y labels.
It does not depends on gca(): If we redo the same from scratch but starting to add a label to the X axis of the UPPER axes, the same issue occurs to re-edit the LAST non-empty label that has been set.


ERROR LOG:
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wrong behavior

HOW TO REPRODUCE THE BUG:
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See hereabove the step-by-step procedure

OTHER INFORMATION:
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Same situation with Scilab 6.0.0-b2+