DHEK19 - Allow to define drawInterpret per mode

*(Depends on #116)*

As `drawInterpret` in `Engine` interpret the subset of Dhek instructions in charge of user interactions with the drawing zone (page), then it will be useful to be able to redefined it for each coming mode:
- Drawing mode (current behaviour, the one activated when opening a new template).
- Duplication mode (#106).
- Multi-selection mode (later).

We need at this step, 
- to move current `drawInterpret` implementation as interpretation instance for the Drawing mode,
- to add mecanism to switch mode, that is to say provide new instance that Dhek should use as `drawInterpret`,
- to switch to Drawing mode when opening PDF.