Implement EVT_MOUSE_CAPTURE_LOST handler in wxSTC

When mouse is captured somwhere in the application, it's recommended
to have implemented a EVT_MOUSE_CAPTURE_LOST handler.
Resetting in the handler flag signalling captured state seems
to be a sufficient action because something like this is implemented
in the native Scintilla and it works fine in SciTE.

Closes #17961.

include/wx/stc/stc.h

@@ -5439,6 +5439,7 @@ protected:
     void OnMenu(wxCommandEvent& evt);
     void OnListBox(wxCommandEvent& evt);
     void OnIdle(wxIdleEvent& evt);
+    void OnMouseCaptureLost(wxMouseCaptureLostEvent& evt);
 
     virtual wxSize DoGetBestSize() const wxOVERRIDE;