Item12873: making proper use of store interface

- attach generated images if possible
- only generate images when required
- delete outdated images 
- lock down image types to gif,jpeg,png and svg ... html5 canvases are pretty different
- allow to generate dynamic size svg images now



git-svn-id: http://svn.foswiki.org/trunk/GnuPlotPlugin@17617 0b4bb1d4-4e5a-0410-9cc4-b2b747904278