Appends a new message source to the end of the section representing this.
Equivalent to calling append_source_with_time with the current time.
It is an error to add a source with an id which already exists. Use
has_source to find out whether there is such a source.
| this |
a App |
| id |
a unique identifier for the source to be added |
| icon |
the icon associated with the source |
| label |
a user-visible string best describing the source |