Replace babel

[set-soft]

• Pros:
  • No more dependency on a 5 MiB (30 MiB uncompressed) rare package.
  • The user can't select a currency that we don't support from the GUI.
• Cons:
  • The currency names displayed in the GUI are always in english (like all the other things in the GUI).
  • If we add KiCost translations we will also need to include the currency names in the list of strings to translate.
  • If we add KiCost translations a more elaborated implementation of format_currency will be needed. Note that Python's locale could be used for this.