Zoom level

Main changes for this pull request : 

- renamed all mentions of dpi to zoom
- dpi is no longer fixed based on the window's size and can be changed dynamically
- drawing controller is listening to mousewheel to zoom-in zoom-out
- minimap is displayed on animated preview when the picture is zoomed in 

Remains to be improved : 
- performance when grid is enabled
- propose implementation with zoom as a tool