locale amounts: consistently use "." as dec point, and " " as thou sep

Always use "." as decimal point, and " " as thousands separator.

Previously,
- for decimal point, we were using
  - "." in some places (e.g. AmountEdit, most fiat amounts), and
  - `locale.localeconv()['decimal_point']` in others.
- for thousands separator, we were using
  - "," in some places (most fiat amounts), and
  - " " in others (format_satoshis)

I think it is better to be consistent even if whatever we pick differs from the locale.
Using whitespace for thousands separator (vs comma) is probably less confusing for people
whose locale would user "." for ts and "," for dp (as in e.g. German).

The alternative option would be to always use the locale. Even if we decide to do that later,
this refactoring should be useful.

closes https://github.com/spesmilo/electrum/issues/2629

electrum/exchange_rate.py

@@ -556,17 +556,24 @@ class FxThread(ThreadJob, EventListener):
         return d.get(ccy, [])
 
     @staticmethod
-    def remove_thousands_separator(text):
-        return text.replace(',', '') # FIXME use THOUSAND_SEPARATOR in util
+    def remove_thousands_separator(text: str) -> str:
+        return text.replace(util.THOUSANDS_SEP, "")
 
-    def ccy_amount_str(self, amount, commas, ccy=None):
+    def ccy_amount_str(self, amount, *, add_thousands_sep: bool = False, ccy=None) -> str:
         prec = CCY_PRECISIONS.get(self.ccy if ccy is None else ccy, 2)
-        fmt_str = "{:%s.%df}" % ("," if commas else "", max(0, prec)) # FIXME use util.THOUSAND_SEPARATOR and util.DECIMAL_POINT
+        fmt_str = "{:%s.%df}" % ("," if add_thousands_sep else "", max(0, prec))
         try:
             rounded_amount = round(amount, prec)
         except decimal.InvalidOperation:
             rounded_amount = amount
-        return fmt_str.format(rounded_amount)
+        text = fmt_str.format(rounded_amount)
+        # replace "," -> THOUSANDS_SEP
+        # replace "." -> DECIMAL_POINT
+        dp_loc = text.find(".")
+        text = text.replace(",", util.THOUSANDS_SEP)
+        if dp_loc == -1:
+            return text
+        return text[:dp_loc] + util.DECIMAL_POINT + text[dp_loc+1:]
 
     async def run(self):
         while True:
@@ -683,7 +690,7 @@ class FxThread(ThreadJob, EventListener):
     def format_fiat(self, value: Decimal) -> str:
         if value.is_nan():
             return _("No data")
-        return "%s" % (self.ccy_amount_str(value, True))
+        return self.ccy_amount_str(value, add_thousands_sep=True)
 
     def history_rate(self, d_t: Optional[datetime]) -> Decimal:
         if d_t is None: