add sendMessage() api for exporting data to saved-messages

[r10s]

sendMessage takes an object with blob, fileName, text as parameters, currently, the message is always sent to the saved-messages chat, for the future, we consider to allow sending to the webxdc's source chat as well, therefore the broader name

TODO:

• settle api, maybe return a promise so we can let the user confirm when it comes to sending to another chat

at webxdc/webxdc-test#9 there is a test for the sendMessage api

targets deltachat/deltachat-core-rust#4350

[github-actions]

To test the changes in this pull request, install this apk:
📦 app-preview.apk

[Simon-Laux]

I think returning a promise makes sense, so apps could show a progress/busy spinner and hide it again once the export is complete.

Also I'm not sure about naming the function sendMessage, does it work now without a file? (code says no), also the naming kinda implies that it is send to the chat. maybe sth like "exportFile" or "exportFileToChat"

[adbenitez]

this looks dangerous, maybe use https://docs.oracle.com/javase/6/docs/api/java/lang/String.html#split%28java.lang.String,%20int%29 to split at most in two strings, and probably also check the case where the filename has no dot "." (no extension) at all

[github-actions]

To test the changes in this pull request, install this apk:
📦 app-preview.apk

[adbenitez]

closing in favor of #2571