Fix a potential (and potentially long standing) bug with numerical input.

The Locale we were using in JNuericTextfield uses the comma ',' as
thousands separator, which breaks reading it with Double.parseDouble...
We get back to using the plain
    "" + val
command to make a string out of a double.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>