fix error if non-latin character was entered in initial value in
data type element

Traceback (most recent call last):
File "/home/beremiz/editors/DataTypeEditor.py", line 575, in OnStructureElementsGridCellChange
value = self.StructureElementsTable.GetValue(row, col)
File "/home/beremiz/editors/DataTypeEditor.py", line 80, in GetValue
return str(value)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-2: ordinal not in range(128)
Traceback (most recent call last):
File "/home/beremiz/editors/DataTypeEditor.py", line 80, in GetValue
return str(value)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-2: ordinal not in range(128)
Traceback (most recent call last):
File "/home/beremiz/editors/DataTypeEditor.py", line 80, in GetValue
return str(value)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-2: ordinal not in range(128)

<?xml version="1.0" encoding="UTF-8"?>
<BeremizRoot URI_location="LOCAL://">
  <TargetType/>
</BeremizRoot>