Part: move icons to create subdirectory

The path to the icons is added to `AppPartGui.cpp`
using `Gui::BitmapFactory().addPath()`, so the icons are found
automatically in their new directories.

Rename icon `Part_CreatePrimitives.svg` to `Part_Primitives.svg`.

src/Mod/Part/BasicShapes/Shapes.py

@@ -85,7 +85,7 @@ if FreeCAD.GuiUp:
             return {'MenuText': Qt.QT_TRANSLATE_NOOP("Part_Tube","Create tube"),
                     'Accel': "",
                     'CmdType': "AlterDoc:Alter3DView:AlterSelection",
-                    'Pixmap': ":/icons/Part_Tube.svg",
+                    'Pixmap': "Part_Tube",
                     'ToolTip': Qt.QT_TRANSLATE_NOOP("Part_Tube","Creates a tube")}
         
         def Activated(self):
@@ -101,3 +101,4 @@ if FreeCAD.GuiUp:
 
 
     FreeCADGui.addCommand('Part_Tube', CommandTube())
+