OmeroCpp: Split omero-client into omero-ice

All slice-generated sources are moved into a separate omero-ice library.  omero-client contains the remainder of the sources and depends on this library.  This reduces the symbol count sufficiently to allow omero-client to link on Windows platforms without triggering any link errors.

`OMERO_API_EXPORTS` also changed to be to either `OMERO_ICE_EXPORTS` and `OMERO_CLIENT_EXPORTS` since these are now two separate libraries.

If this works correctly, it should be fairly simple to split and link two separate libraries with scons for 5.0.4.

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Testing:

- jobs should remain green for Linux/Mac
- exported symbols should be unchanged on [Linux]/{Mac} `nm [-D]{-g}`, though they will be split between the two libs
- code should compile and link under Windows
- I'll enable building under Windows; it may not be green initially since the CI has never built it before