feat(ffi): Add DC_STR_CANT_DECRYPT_OUTGOING_MSGS define

[iequidoo]

As @gerryfrancis said,

FYI, I have gotten this system message after I created new DC profiles (for testing), but the device has been the same since. Although I know what this is about, it might confuse other DC users in similar situations...

So maybe making less assumtions about what the user does wrong is better. Just in case, the previous discussion of this stock string: #5176 (comment)

[link2xt]

This looks good but if we merge this we should also change the string in the Android repo because it is the string that is actually used and gets translated. Core fallback is normally not used at all.

[r10s]

tbh, not sure if the new wording is better.

the "some outgoing messages" seems more vague than "each other's outgoing messages". and the wording adopts to rarer cases (as @gerryfrancis was using the same device during testing), however probably weakens the string for the majority of cases (user really setting up a second device wrongly).

all in all i found the old wording more on point and actionable ("to fix that")

but yeah, it is a minor one and if we think the rarer cases happen often enough outside artificial testing scenarios, we can go for it and i am also fine with it.

for translations: in fact, the string is not yet translated at all yet (what a luck!, so we can do changes without annoying translators :), also the corresponding define in deltachat.h is missing

[iequidoo]

I agree with @r10s that the existing phrasing is probably better for the most users, but just in case tried to improve it once more

[link2xt]

"Could not decrypt outgoing messages that may be from Delta Chat on your other device." can be read as "Could not decrypt ... on your other device."

So we have these options:

• ⚠️ It seems you are using Delta Chat on multiple devices that cannot decrypt each other's outgoing messages. To fix this, on the older device use \"Settings / Add Second Device\" and follow the instructions. (original)
• ⚠️ Could not decrypt some outgoing messages. If you are using Delta Chat on multiple devices, on the older device use \"Settings / Add Second Device\" and follow the instructions. (before force-push)
• ⚠️ Could not decrypt outgoing messages that may be from Delta Chat on your other device. If so, to fix this, on the older device use \"Settings / Add Second Device\" and follow the instructions. (after force-push)

[iequidoo]

"Could not decrypt outgoing messages that may be from Delta Chat on your other device." can be read as "Could not decrypt ... on your other device."

I don't think so, even Google Translate feels the difference (when translating to Spanish f.e.) if i put commas as follows:
⚠️ Could not decrypt outgoing messages, that may be from Delta Chat, on your other device. If so, to fix this, on the older device use \"Settings / Add Second Device\" and follow the instructions.
But another option is
⚠️ Could not decrypt outgoing messages that may be from your other device's Delta Chat. If so, to fix this, on the older device use \"Settings / Add Second Device\" and follow the instructions.