Make iscurrent attribute read-only.

[Henning Eggers]

Now that we have a POTemplate.setActive method, the iscurrent attribute should be made read-only and setActive exported through the webservice. Making icurrent read-only forces the use of setActive which will update the suggestive potemplate cache when needed.
Most likely the bulk of the work to fix this is in updating all the tests that set iscurrent directly. That could probably be done in a first mechanical branch.