Project Tab Panels do not display when required fields partially disabled.

In a situation where there are multiple field layout schemes for a project, if only one or more of the layout schemes have a required field hidden, then the tab panels won't appear.

Thus in the default install, where all the tab panels requires fixVersions, if fixVersions is disabled in any of the field layout schemes, then none of the them will appear.

I'm not sure if it should work this way, since there could still be issues with other field layouts which could still be relevant. Thus I think it's a bug, but could be convinced otherwise.

We should perhaps document "required fields" for each portlet/panel we write as to not cause confusion.