Change Polish key export_png to export_img (as already changed in other locales); have emailed for confirmation of accuracy of translation, awaiting reply

git-svn-id: http://svg-edit.googlecode.com/svn/trunk@2659 eee81c28-f429-11dd-99c0-75d572ba1ddd
Possible todo to support server-side saving of exports as well

git-svn-id: http://svg-edit.googlecode.com/svn/trunk@2660 eee81c28-f429-11dd-99c0-75d572ba1ddd