Adds a validation of min and max scale values.
Min can't be more than max anymore.
Fix a bug in SIF that made possible to validate an invalid input. If the
user clicked Ok with an invalid input and then used the cross of the
window to close it, the invalid input was saved...
Change to error icon; update translations
We must escape ' with another '
Merge pull request #491 from agueganno/scale-validation

* Make sure min scale is less than max scale
* Fix a bug in SIF that made it possible to validate an invalid input

@gpetit Please check #443.