improve the videocalls: open in internal webview

[adbenitez]

• integrate https://github.com/deltachat/calls-webapp
• use new core calls API
• add permanent notification on outgoing call
• add ringing notification on incoming call
• wait for add call ringing API chatmail/core#6650 to be merged
• wait for a new core to be tagged including the new calls API
• update changelog
• make sure, multidevice is working: calls may ring on several devices, once accepted on one, the others should stop ringing

counterpart of deltachat/deltachat-ios#2638

[adbenitez]

the dialog that appears when inviting someone says that you need to have an app installed, we probably need a new string

[github-actions]

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

[r10s]

the dialog that appears when inviting someone says that you need to have an app installed, we probably need a new string

for now, we can just not use the string videochat_invite_user_hint

[r10s]

great that you figured out all that!

if needed, we can merge that roughly as is to move forward.

the only thing we should necessarily change is to not show the call option by default, maybe we need a switch for that, as we introduced a default string, but any other way that comes in mind would do as well

[github-actions]

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

[github-actions]

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