Fixes#260
Description of the changes:
prevent UnitConverterViewModel to reset values when users click on update rates.
recompute UnitConverter's caches (m_ratioMap and m_categoryToUnits) once rates are updated (but check first if the user did/didn't change the category)
How changes were validated:
Manually tested with fake currency rates (HTTP responses modified on the fly via FiddlerCore)
Verified that it works no matter the selected field (From or To)
Verified that the currencies selected are kept after a refresh