qt: Use locale-specific number formatting

- Change bitcoinamountfield to use locale-specific number format
- Change bitcoinunits to show and parse locale-specific numbers
- If a language/territory is selected in options, this is set as default locale (overrides system locale)

Examples:
- US: 123,456.78 μBTC
- Dutch: 123.456,78 μBTC
- German (Switzerland): 123'456.78
...

Fixes #3887