Add the 3i SlideBook jar to the list of jars if present

[ehrenfeu]

Optionally load the SlideBook6Reader.jar if the file is present.

This adds (native) support for loading 3i's .sld files.

The JAR can be fetched from the corresponding SlideBook Fiji update site - please make sure to remove the timestamp suffix.

[ehrenfeu]

(Just Cc'ing my colleagues @lguerard @cellkai and @sebherbert to keep them in the loop)