Various icon and cursor improvements #4738

- The mouse cursor now is an open hand when hovering over something
  which can be panned, e.g. the red square in the Navigator panel. It is
  a crosshair when hovering over the main preview.
- When you pan the red square or the main preview, it's a closed hand.
- Now the cursor changes to a crop icon when you're in crop mode.
- The cursor changes as needed when you want to resize a crop edge or
  corner.
- The cursor changes when you're in straighten mode.
- The curve "add adjustment point" and "edit node in/out values" icons
  now contain a node in the icon and are more representative of what
  they do.
- The small reset/undo icon is made a bit clearer.

rtgui/cropwindow.cc

@@ -1251,7 +1251,7 @@ void CropWindow::updateCursor (int x, int y)
                 if (onArea (CropObserved, x, y)) {
                     newType = CSMove;
                 } else {
-                    newType = CSHandOpen;
+                    newType = CSCrosshair;
                 }
             } else if (tm == TMSpotWB) {
                 newType = CSSpotWB;