Return wxString for gfx importer messages

Returning a reference to a temporary value crashes when the value is
deallocated before use.  Passing a full string fixes the crash and
allows NVRO on the temporary value.

pcbnew/import_gfx/dxf_import_plugin.h

@@ -217,7 +217,7 @@ public:
     /**
      * @return the list of messages in one string. Each message ends by '\n'
      */
-    const wxString& GetMessages() const override
+    wxString GetMessages() const override
     {
         return m_messages;
     }

pcbnew/import_gfx/graphics_import_plugin.h

@@ -118,7 +118,7 @@ public:
      *
      * @return the list of messages in one string. Each message ends by '\n'
      */
-    const virtual wxString& GetMessages() const = 0;
+    virtual wxString GetMessages() const = 0;
 
 protected:
     ///> Importer used to create objects representing the imported shapes.

pcbnew/import_gfx/graphics_importer.h

@@ -83,7 +83,7 @@ public:
      *
      * @return the list of messages in one string. Each message ends by '\n'
      */
-    const wxString& GetMessages() const
+    wxString GetMessages() const
     {
         return m_plugin->GetMessages();
     }

pcbnew/import_gfx/svg_import_plugin.h

@@ -52,7 +52,7 @@ public:
     /**
      * @return the list of messages in one string. Each message ends by '\n'
      */
-    const wxString& GetMessages() const override
+    wxString GetMessages() const override
     {
         return m_messages;
     }