FIX: shutdown GUI_App after confirm dialog

Change-Id: Ie379e9e764f43be1592a74b929bfbfc1d2d41a49
Signed-off-by: Stone Li <stone.li@bambulab.com>

src/slic3r/GUI/GUI_App.hpp

@@ -280,6 +280,7 @@ public:
     // Process command line parameters cached in this->init_params,
     // load configs, STLs etc.
     void            post_init();
+    void            shutdown();
     // If formatted for github, plaintext with OpenGL extensions enclosed into <details>.
     // Otherwise HTML formatted for the system info dialog.
     static std::string get_gl_info(bool for_github);