Build the Design tab on first use instead of at startup

DesignPanel's constructor creates several hundred controls and its own GL
canvas, which every launch paid for whether or not the user ever opened the
tab. The notebook page is now an empty placeholder and the panel is built
into it the first time the tab is selected, so m_design_panel stays null
until then.

This also lifts the ordering constraint that had pushed the plater's
background colour and Hide() below the panel construction; they go back
where they were.

src/slic3r/GUI/MainFrame.hpp

@@ -387,6 +387,9 @@ public:
     PrintHostQueueDialog* printhost_queue_dlg() { return m_printhost_queue_dlg; }
     Plater*               m_plater { nullptr };
 #ifdef SLIC3R_CAD
+    // The tab page is the placeholder; m_design_panel stays null until the tab is first
+    // selected, so everything the Design panel builds stays off the startup path.
+    wxPanel*              m_design_page { nullptr };
     DesignPanel*          m_design_panel { nullptr };
 #endif
     //BBS: GUI refactor