Forward IME messages to ScintillaWX and handle them

The handling code is copied from ScintillaWin

Closes #18759

include/wx/stc/stc.h

@@ -5493,6 +5493,10 @@ protected:
     void NotifyChange();
     void NotifyParent(SCNotification* scn);
 
+#ifdef __WXMSW__
+    virtual WXLRESULT MSWWindowProc(WXUINT nMsg, WXWPARAM wParam, WXLPARAM lParam) wxOVERRIDE;
+#endif // __WXMSW__
+
 private:
     wxDECLARE_EVENT_TABLE();
     wxDECLARE_DYNAMIC_CLASS(wxStyledTextCtrl);