Help message in statusbar

Help message is not updated when mode changes.

- Currently: Always keep Draw mode message ("Cliquer pour dessiner, ..."), even in Duplication or Selection mode.
- Expected: Message specific to mode set each time mode is activated. Means no change at app launch as Draw mode is the one activated when it starts, but should not be the same when other mode is activated.

**For Duplication Mode:**
*French*
> Glissé-déposé une zone pour la dupliquer.
*English*
> Drag&drop area to duplicate it.

**For Selection Mode:**
*French*
> Cliquer pour débuter une sélection, relâcher pour terminer. META + clic pour modifier la sélection.
*English*
> Click to start selection, release to finalize. META + click to update selection.

(META = to be replaced CTRL by default, and by CMD on Mac OS X)