Captions of the media wiki images are imported only as alt texts for the XWiki images, titles should be set as well

When MEDIAWIKI-4 was done, the image caption was chosen to be set as alt text of the imported XWiki image.
It should be set as title as well, in order to give access to the users to this text, to allow them to read it (alt text "hides" these caption texts, they only get displayed on the screen if the image cannot be loaded, which is not the purpose of the caption text in Media wiki).

Note that this would be coherent also with what Media wiki does for images that are set to not have links, sets both alt and title to the caption value, see the last example here https://www.mediawiki.org/wiki/Help:Images#Altering_the_default_link_target .