Display duplication cursor earlier

While in Draw mode, update mouse cursor to Duplication one (the one of Duplication mode) as soon as key modifier (ALT) is detected over drawing zone, not waiting mouse to be pressed.

**Summary:**
- *(todo)* Event for cursor change = pointer over drawing zone (mouse move?) + ALT
- *(todo)* Event to rollback cursor to default one of Draw mode = ALT release OR mouse exit drawing zone.
- *(already impl'ed)* Event to start duplication mode = ALT + mouse pressed on drawing zone