Gui: better icon for the Origin object

During the LinkMerge, a new icon
`Std_CoordinateSystem.svg` was used
for the `App::Origin` object that appears in
`PartDesign::Body`. However, this icon
doesn't look as good as the original `CoordinateSystem.svg`.

This commit renames the old icon to the new name. The newer icon
is kept as an alternate version `Std_CoordinateSystem_alt.svg`.

In addition, a new icon `Std_Placement.svg`
is provided for the `App::Placement` object, which previously
was also using `CoordinateSystem.svg`.
In this way, `App::Placement` and `App::Origin`
use different icons and there is no confusion
in the tree view.

src/Gui/ViewProviderPlacement.cpp

@@ -73,7 +73,7 @@ ViewProviderPlacement::ViewProviderPlacement()
     pcRoot->unref();
     pcRoot = newRoot;
     pcRoot->ref();
-    sPixmap = "CoordinateSystem";
+    sPixmap = "Std_Placement";
 
     OnTopWhenSelected.setValue(1);
 }