Light(Preview) theme-Fix for missing alternate row colors for Tree [MAC]

Tree Viewers had alternate row colors if they had coloumns, in Light
Theme. This was missing in Light(Preview) theme. This has been fixed
with this change by making swt-lines-visible (Tree Property) to true.

bundles/org.eclipse.ui.themes/css/e4_preview_mac.css

@@ -104,7 +104,6 @@ CTabFolder Canvas {
 }
 
 .View Composite,
-.View Composite Tree,
 .View Composite Label,
 .View ToolBar,
 .View Group,
@@ -126,6 +125,11 @@ CTabFolder Canvas {
  background-color: #f8f8f8;
 }
 
+.View Composite Tree{
+ background-color: #f8f8f8;
+ swt-lines-visible: true;
+}
+
 .View TitleRegion{
  background-color:#f8f8f8;
 }