Modified Preview Canvas

- Now the Preview can show free space around the image (the image's
corner will coincide with the center of the preview area)
- Editing objects can now be manipulated in this free space
- The editing mechanism has been split : it was completely handled in
rtengine before, now rtengine still handle the pipette's data provider,
but rtgui now handle the objects data provider.
- Bugfix: when using coarse rotate in the Editor panel, the Gradient
widgets are now correctly displayed

rtengine/curves.h

@@ -28,7 +28,7 @@
 #include "../rtgui/mydiagonalcurve.h"
 #include "color.h"
 #include "procparams.h"
-#include "editbuffer.h"
+#include "pipettebuffer.h"
 
 #include "LUT.h"