Fix of the final nasty flashcard bug (and more image related small bugs)! Flashcard mode now completely works!

Code cleanup.
Code robustness++
Small API change

Change in theme requirements: the background surrounding active areas is called 'mode_background' (eg written_background) instead of 'mode_box'.

The combination of mode + image_box and mode_image + box caused a nasty naming conflict (which was part of the flashcard problem).

The techbase page has been updated.