The code was more complicated than necessary, with the base class
providing both virtual GtkOnCellChanged() and GtkOnTextEdited() that
were both overridden to achieve the same thing, namely customizing how
the value entered by user is converted to wxVariant, in different
derived classes.
Make GtkOnTextEdited() non-virtual and remove GtkOnCellChanged()
completely and add a new simple GtkGetValueFromString() which is called
from GtkOnTextEdited() to do the conversion.
This removes the existing code duplication and will make it simpler to
modify this code in the future, without changing the behaviour.
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