imageviewer updates

- new zoom icons
- resetview option
- properly track pushbutton checked state
- add preferences options for selecting background and box color

tools/lammps-gui/chartviewer.cpp

@@ -53,7 +53,7 @@ int ChartWindow::get_step() const
 {
     if (charts.size() > 0) {
         auto *v = charts[0];
-        return (int)v->get_step(v->get_count()-1);
+        return (int)v->get_step(v->get_count() - 1);
     } else {
         return -1;
     }