Add caf::PdmFieldReorderCapability to the child array fields holding
RimCase objects:
- RimEclipseCaseCollection::cases (grid models in Project Tree)
- RimCaseCollection::reservoirs (statistics case groups)
- RimGeoMechModels::m_cases (geomechanical models)
This enables the built-in move up/down tree item controls for
reordering cases, matching the pattern already used for e.g.
RimSurfaceCollection and RimMultiPlot.