this PR adds a function that can be used to create any QR code, in a raw
form.
this can be used to create add-contact as well as add-second-device QR
codes (eg. `dc_create_qr_svg(dc_get_securejoin_qr())`) - as well as for
other QR codes as proxies.
the disadvantage of the rich-formatted QR codes as created by
`dc_get_securejoin_qr_svg()` and `dc_backup_provider_get_qr_svg()` were:
- they do not look good and cannot interact with UI layout wise (but
also tapping eg. an address is not easily possible)
- esp. text really looks bad. even with
[some](e5dc8fe3d8)
[hacks](https://github.com/deltachat/deltachat-android/pull/2215) it
[stays buggy](https://github.com/deltachat/deltachat-ios/issues/2200);
the bugs mainly come from different SVG implementation, all need their
own quirks
- accessibility is probably bad as well
we thought that time, SVG is a great thing for QR codes, but apart from
basic geometrics, it is not.
so, we avoid text, this also means to avoid putting an avatar in the
middle of the QR code (we can put some generic symbol there, eg.
different ones for add-contact and add-second-device).
while this looks like a degradation, also other messengers use more raw
QR codes. also, we removed many data from the QR code anyway, eg. the
email address is no longer there. that time, sharing QR images was more
a thing, meanwhile we have invite links, that are much better for that
purpose.
in theory, we could also leave the SVG path completely and go for PNG -
which we did not that time as PNG and text looks bad, as the system font
is not easily usable :) but going for PNG would add further challenges
as passing binary data around, and also UI-implemtation-wise, that would
be a larger step. so, let's stay with SVG in a form we know is
compatible.
the old QR code functions are deprecated.
Restart the IO scheduler if needed to make the new config value effective (for `MvboxMove,
OnlyFetchMvbox, SentboxWatch` currently). Also add `set_config_internal()` which doesn't affect
running the IO scheduler. The reason is that `Scheduler::start()` itself calls `set_config()`,
although not for the mentioned keys, but still, and also Rust complains about recursive async calls.
