config: introduce ConfigVars

A new config API is introduced, and ~all of the codebase is adapted to it.
The old API is kept but mainly only for dynamic usage where its extra flexibility is needed.

Using examples, the old config API looked this:
```
>>> config.get("request_expiry", 86400)
604800
>>> config.set_key("request_expiry", 86400)
>>>
```

The new config API instead:
```
>>> config.WALLET_PAYREQ_EXPIRY_SECONDS
604800
>>> config.WALLET_PAYREQ_EXPIRY_SECONDS = 86400
>>>
```

The old API operated on arbitrary string keys, the new one uses
a static ~enum-like list of variables.

With the new API:
- there is a single centralised list of config variables, as opposed to
  these being scattered all over
- no more duplication of default values (in the getters)
- there is now some (minimal for now) type-validation/conversion for
  the config values

closes https://github.com/spesmilo/electrum/pull/5640
closes https://github.com/spesmilo/electrum/pull/5649

Note: there is yet a third API added here, for certain niche/abstract use-cases,
where we need a reference to the config variable itself.
It should only be used when needed:
```
>>> var = config.cv.WALLET_PAYREQ_EXPIRY_SECONDS
>>> var
<ConfigVarWithConfig key='request_expiry'>
>>> var.get()
604800
>>> var.set(3600)
>>> var.get_default_value()
86400
>>> var.is_set()
True
>>> var.is_modifiable()
True
```

electrum/exchange_rate.py

@@ -23,11 +23,6 @@ from .simple_config import SimpleConfig
 from .logging import Logger
 
 
-DEFAULT_ENABLED = False
-DEFAULT_CURRENCY = "EUR"
-DEFAULT_EXCHANGE = "CoinGecko"  # default exchange should ideally provide historical rates
-
-
 # See https://en.wikipedia.org/wiki/ISO_4217
 CCY_PRECISIONS = {'BHD': 3, 'BIF': 0, 'BYR': 0, 'CLF': 4, 'CLP': 0,
                   'CVE': 0, 'DJF': 0, 'GNF': 0, 'IQD': 3, 'ISK': 0,
@@ -577,29 +572,29 @@ class FxThread(ThreadJob, EventListener):
             if self.is_enabled():
                 await self.exchange.update_safe(self.ccy)
 
-    def is_enabled(self):
-        return bool(self.config.get('use_exchange_rate', DEFAULT_ENABLED))
+    def is_enabled(self) -> bool:
+        return self.config.FX_USE_EXCHANGE_RATE
 
-    def set_enabled(self, b):
-        self.config.set_key('use_exchange_rate', bool(b))
+    def set_enabled(self, b: bool) -> None:
+        self.config.FX_USE_EXCHANGE_RATE = b
         self.trigger_update()
 
     def can_have_history(self):
         return self.is_enabled() and self.ccy in self.exchange.history_ccys()
 
     def has_history(self) -> bool:
-        return self.can_have_history() and bool(self.config.get('history_rates', False))
+        return self.can_have_history() and self.config.FX_HISTORY_RATES
 
     def get_currency(self) -> str:
         '''Use when dynamic fetching is needed'''
-        return self.config.get("currency", DEFAULT_CURRENCY)
+        return self.config.FX_CURRENCY
 
     def config_exchange(self):
-        return self.config.get('use_exchange', DEFAULT_EXCHANGE)
+        return self.config.FX_EXCHANGE
 
     def set_currency(self, ccy: str):
         self.ccy = ccy
-        self.config.set_key('currency', ccy, save=True)
+        self.config.FX_CURRENCY = ccy
         self.trigger_update()
         self.on_quotes()
 
@@ -608,10 +603,10 @@ class FxThread(ThreadJob, EventListener):
         loop.call_soon_threadsafe(self._trigger.set)
 
     def set_exchange(self, name):
-        class_ = globals().get(name) or globals().get(DEFAULT_EXCHANGE)
+        class_ = globals().get(name) or globals().get(self.config.cv.FX_EXCHANGE.get_default_value())
         self.logger.info(f"using exchange {name}")
         if self.config_exchange() != name:
-            self.config.set_key('use_exchange', name, save=True)
+            self.config.FX_EXCHANGE = name
         assert issubclass(class_, ExchangeBase), f"unexpected type {class_} for {name}"
         self.exchange = class_(self.on_quotes, self.on_history)  # type: ExchangeBase
         # A new exchange means new fx quotes, initially empty.  Force
@@ -689,4 +684,4 @@ class FxThread(ThreadJob, EventListener):
         return self.history_rate(date)
 
 
-assert globals().get(DEFAULT_EXCHANGE), f"default exchange {DEFAULT_EXCHANGE} does not exist"
+assert globals().get(SimpleConfig.FX_EXCHANGE.get_default_value()), f"default exchange {SimpleConfig.FX_EXCHANGE.get_default_value()} does not exist"