Appends msg to the source with id source_id of this.
The messaging menu might not display this message immediately if other messages are queued before this one.
If source_id has a count associated with it, that count will be increased by one.
If source_id is null, msg won't be associated with a source.
| this |
a App |
| msg |
the Message to append |
| source_id |
the source id to which |
| notify |
whether a notification bubble should be shown for this message |