- it's now possible to start a drawing with a pen and use fingers to zoom
or scroll without interacting with the current drawing;
- it's now possible to draw with a finger and them zoom with two fingers.
This patch makes a clear separation between the way to draw and the editing stuff.
It adds a class DrawEditor which should be extended in order to create new drawing tools.
As an example, the ink tool has been rewritten in order to use it.