The Navigator uses the same hand icon regardless whether panning
the red square or not, this branch tries to fix that, #4738
Includes a cleanup of icon names, i.e. the crosshair should not be
called the hand.
- 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
