qt: Format "Balance" amount using commas #3887

I have my client configured to display amounts in mBTC. The Balance on the Overview page is hard to parse because it doesn't format the amount using commas for every three positions, as is standard for maximally-human-readable display of large numbers. This issue also applies to the uBTC display option and possibly to the BTC display option, but I don't have enough in my wallet to test that one. Note how unhelpful this is:

1234567.89000 mBTC

There is a wiggle here in that some cultures use a period as a delimiter instead of a comma, but not in English, and I see that there are different locale files for other languages, so there's probably an i18n solution available.

I would be in favour of using a thin space as a seperator. This is less confusing for people from regions where commas are used as decimal seperator instead of thousand seperator.

@leofidus have you seen #3893? The idea is to simply use the number formatting (group and decimal seperator) of the user locale. Let's not reinvent any wheels.