Import backups in XML format

It seems(at least for me) that Signal has changed something that makes it no longer allow SMS Backup+ to work properly. That being said, this isn't your issue, nor is it your job to attempt to support other SMS apps.

What I'd like is to request an SMS import feature, to import exported SMS XML files(which are pretty standard) and backup those to the gmail account. This would require me to do my own backups, but I like the way messages get backed up to Gmail, so that's not a problem.

Related: #841

Sound somehow similar to #749

@Woi

Kinda the opposite of that; it's restoring from a local file, rather than backing up to one.

On the other hand, it could be argued that both halves form a single issue.