Show the Design tab's reference planes as separate labelled squares around the origin

The XY, XZ and YZ planes no longer cross through the bed. Each is a small square in its axis
colour, set off from the axes into the corner that faces the default front view, with its name
written in the plane. Dash-dot axes run between the squares and replace the bed's axis triad
while they show. Hovering a plane greys it and selecting one makes it solid, and picking a
solid face now clears a previously picked plane.

src/slic3r/GUI/CAD/DesignCanvas.cpp

@@ -1037,6 +1037,11 @@ void DesignCanvas::set_on_datum_base_picked(std::function<void(int)> cb)
     m_sketch_tool.on_datum_base_picked = std::move(cb);
 }
 
+void DesignCanvas::set_selected_base(std::function<int()> cb)
+{
+    m_sketch_tool.selected_base = std::move(cb);
+}
+
 void DesignCanvas::set_on_sketch_exit(std::function<void()> cb)
 {
     m_sketch_tool.on_exit = std::move(cb);