Use different image sizes in preview mode than in full screen for the Gallery Macro

For performance reasons, it would be smart for the Gallery Macro to display thumbnails (reduced size versions of the displayed images) while in preview mode, while in full screen it should display the full sized image.

It should be trivial to implement and it would avoid loading a high resolution image (i.e. 3000+px) just to display its thumbnail.

XWiki`s image resizing parameters should be reused by the Gallery Macro behind the scenes, to easily achieve this.