DHEK20 - Duplicate area

*(Depends on #117)*

There we add a new mode, with its specific `drawInterpret` which will handle following user interaction.

1. User click (mouseDown+mouseUp) an existing area in drawing zone (page): duplication operation starts, a temporary area is instantiated (just as when creating a new area in Drawing mode). 
  - This *being-created* area is placed under mouse cursor: the area center is right under mouse cursor.
  - Area width & height are those of original area (the one duplicated).
2. While the user moves the mouse cursor within drawing zone (page), the *being-created* area follows this move.
3. User perform second click, the *being-created* area is dropped (stops following mouse cursor), and it's finally added to area list).