for now the value is converted to a string and then printed. in many
cases the values are too wide or double values. so it make sense
to set a custom value text instead of the default behavior.
this patch suggests to add a method
"fn text_value(mut self, text_value: String)"
to the Bar, which allows to override the value printed in the bar
Signed-off-by: Ben Fekih, Hichem <hichem.f@live.de>