Remove sphinx.environment.BuildEnvironment and sphinx.util.relative_uri

This feature does not work for now
Fix docstrings of omero.callbacks
Fix omero __init__.py header
Fix docstrings in omero.gateway

Should remove all warnings when building the sphinx-api
Fix docstring of omero.install.versions
Fix docstring in omero.util.import_candidates
Fix docstring in omero.util.populate_roi
Fix docstring in omero.util
Merge pull request #2910 from sbesson/rebased/develop/sphinxapi_compilation

Fix warnings in Sphinx API generation (rebased onto develop)