wxEVT_CREATE and wxEVT_ACTIVATE is not being called on the main frame

on application start-up, at least not on Windows.
wxEVT_CREATE was called for some control deep in the Plater, however
the event was delivered to the main frame and only for slicer, not
for G-code viewer. Thus the callbacks for 3D Mouse were not registered
for and the 3D mouse did not work on Windows.

Fixed by calling the callback registration from the first execution
of the Idle function.

src/slic3r/GUI/GUI_App.hpp

@@ -143,7 +143,7 @@ private:
 	size_t m_instance_hash_int;
 
     // parameters needed for the after OnInit() loads
-    struct AFTER_INIT_LOADS
+    struct AfterInitLoads 
     {
         std::vector<std::string>    m_load_configs;
         DynamicPrintConfig          m_extra_config;
@@ -272,7 +272,7 @@ public:
     PresetUpdater*  preset_updater{ nullptr };
     MainFrame*      mainframe{ nullptr };
     Plater*         plater_{ nullptr };
-    AFTER_INIT_LOADS m_after_init_loads;
+    AfterInitLoads  after_init_loads;
 
 	PresetUpdater* get_preset_updater() { return preset_updater; }