api: create QR codes from any data

[r10s]

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 hacks it stays buggy; 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.

[link2xt]

JSON-RPC API is missing, but fine to merge without it. Can be added in a separate PR then.

[r10s]

+1 (i left out jsonrpc on purpose, let's see if the api is sufficient :)