Support explicit currency setting per document (allocatz/worthz) file

[reedes]

At present, both FlowAllocator and FlowWorth assume all accounts are in a common currency, where the currency symbol (and formatting method) is chosen from the locale.

Provide a way for the user to select an alternative currency which will be stored in the DisplaySettings of the document (allocatz/worth) file.

Implementation detail: the unused CurrencyCodePicker in FlowAllocatorApp could be used. Relocate it to FlowUI.

Additional implementation detail: on file load, or when user changes the explicit currency, see if it's possible to make an app-wide change to the locale in the environment to reflect the user's preference. Ideally, the formatters should automatically pick this up.