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).

Fixes #3887

(cherry picked from commit
laanwj/bitcoin@db0b8f3a2e96bc7c7aff95c9ade65be33dd27209,
bitcoin/bitcoin#3893)
qt: Add tests for bitcoin units parsing/formatting

Tests various locales, as well as variants with and without
decimals group separators.

(cherry picked from commit
laanwj/bitcoin@9ce31063b8c7e2ecac4087f2487d1c4da5abe29d,
bitcoin/bitcoin#3893)
use system local for formatting instead of selected locale
convert number formatting unit test to dogecoin
Merge pull request #400 from leofidus/1.7-localformat

1.7 number format according to system local