Reset the CAD document with the project, and track its changes

New Project and Open Project went through Plater::priv::reset, which
drops ModelObjects but not the Model-level recipe, and never touched
the Design panel's document at all — so the previous design stayed
loaded and its next edit wrote itself into the new project.

A design that has not been committed to the plate has no ModelObjects,
so the project also read as clean: no autosave, and no unsaved-changes
prompt before the reset threw the design away.

src/slic3r/GUI/CAD/DesignPanel.hpp

@@ -51,6 +51,7 @@ public:
     explicit DesignPanel(wxWindow* parent);
     void on_tab_shown();        // re-sync bed to the active printer when the Design tab is activated
     void on_tab_hidden();       // another tab took over: take the viewport status line down with us
+    void clear_document();      // New Project / Open Project: drop the document with the project
     // Rebuild off the UI thread (progress dialog only if it turns out to be slow), so a feature
     // op on a heavy imported solid does not freeze the window. Returns m_doc.recompute()'s result.
     // Push the document's recipe into the Model so ANY save path persists it (snaporca-vjk5).