The qtmultimedia-based qrreader has the concept of "strong_count":
before the scanner returns a decoded qr code result, it waits until
it has seen at least "strong_count" (e.g. 10) frames in which the qr code was seen and successfully decoded.
I think the idea might have been to reduce false positives, mis-decoding qr codes from bad frames.
However in practice it makes scanning even moderately sized qr codes really difficult for the user:
it takes several seconds (at least on my laptop cam) to obtain enough "clear" frames that count into the strong_count.
So I am lowering the strong_count to 2, down from CAMERA_FPS/3,
which makes it easier to scan, and I still haven't seen false positives even with this value.
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
```
SomberNight