Timesheet entry client property

[gsvarovsky]

At present it's not possible to know for sure what client a timesheet entry is for.

If the timesheet is assigned to exactly one project then it might be inferred that its entries are for that project's account in the role of a client. However, since a timesheet may be attached to zero or many projects, any entry could be for any of those projects, or none.

This impacts Prejournal integration, because it requires a single client for each worked-time movement.