Zanshin: show datasource in inbox page model.

Summary:
This is another one of those task details that are currently invisible.

However now that I'm testing it, I'm having second thoughts. In normal
usage all inbox tasks come from the same (default) datasource. This is
only useful when having multiple datasources (e.g. during a resource migration),
and when having doubts about where the tasks come from.
Maybe a tooltip (or a details dialog) would actually be enough for this use case. On the other hand it makes
the rendering somewhat consistent with the workday page ;-)

Well, one use case that this would cover nicely is people who want a
full private/work separation, including a different collection on their
private and work machines, while still seeing everything in both...

Test Plan: Starting zanshin and going to inbox, notice the second line showing the datasource name.

Reviewers: ervin

Reviewed By: ervin

Subscribers: Zanshin

Differential Revision: https://phabricator.kde.org/D16210