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/toolbar.cc

@@ -46,7 +46,7 @@ ToolBar::ToolBar () : showColPickers(true), listener (nullptr), pickerListener(n
 
     pack_start (*wbTool);
 
-    showcolpickersimg.reset(new RTImage("color-picker-show.png"));
+    showcolpickersimg.reset(new RTImage("color-picker-bars.png"));
     hidecolpickersimg.reset(new RTImage("color-picker-hide.png"));
 
     colPickerTool = Gtk::manage (new Gtk::ToggleButton ());