Support in OmeroWEB to obtain the thumbnail of any slice of an image.

The new call render_slice_thumbnail is also able to set up the default rendering settings.
Also a small fix in the build file for OmeroWeb, the default=install was incorrect.
Function _imagekey converts z and t to string internally:

https://github.com/openmicroscopy/openmicroscopy/blob/develop/components/tools/OmeroWeb/omeroweb/webgateway/webgateway_cache.py#L566