lua: add a function to drop the thumbnail for an image

to be called to when an image is modified externally

note that there is no way to regenerate the thumbnail. The images to keep
in cache are under the GUI's responsability and it will do a better job at
figuring out who to keep
lua documentation : make documentation generation stable again
lua : fix an error in j2k handling
lua : restore the lost "read" and "write" attributes in documentation
lua : fix bug #10123 : crash when using presets with lua storages

This is due to the addition of versioning to storage parameters

lua currently always reports a version of 0
(which is fine since lua can't store storage parameters at this point)