colorbar label format string does not work

[Angus Yeh]

the use of "label_format_string" key in colorbars.json does not work properly:

colorbars.json edited to include "label_format_string" does not have an effect in UI, it was turned into unicode when loaded, so mechanism failed to recognised it as string

An side-effect of this is that that format string will not be saved properly as TIM is closed.