Replace ~/omero with standard OS-dependent paths

[manics]

Replaces ~/omero with "standard" paths according to each operating system: https://pypi.org/project/appdirs/

Closes #210

[manics]

On OS X this results in:

• get_omero_userdir: ~/Library/Application Support/OMERO.cli/5
• get_omero_user_cache_dir: ~/Library/Caches/OMERO.cli/5

[manics]

Is this considered a breaking change that should wait for 6.0.0?. If so this could be split so only omero.util.get_omero_user_cache_dir() which was added in #162 uses the new paths

[sbesson]

Few immediate thoughts:

• agreed the changes are not breaking the API although we might want to review the impact of such a change
• I suspect we will need to consider the case where downstream applications are accessing application data and moving away from ~/omero would break the expectations. In that sense, I think the delineation proposed in Replace ~/omero with standard OS-dependent paths #233 (comment) is probably an acceptable compromise as anything stored in a cache directory should be considered as potentially transient
• more generally, do you have some mechanism that would allow the migration of data between the old user/cache directory and the new one
• the application specific aspect of the paths also raises the questions of the unification with other OMERO clients e.g. OMERO.insight is using ~/omero/log

[manics]

I'll rebase on top of #240