cmake: Split omero-client into omero-ice

All slice-generated sources are build 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.

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
- 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