Allow cancelling a scan session.

- perform scanning in a thread
 - change the label of the scan button
 - trigger cancel with the same button and with the ESC key
 - process all GUI updates in asynchronous mode.

Do not use a top window as parent of popups:
 - If a top window goes away in an application with multiple instances of
 XInsaneWidget, any call to a scanner widget leads to a crash.

Minor changes.

XS7.cpp

@@ -126,7 +126,7 @@ void XS7::OnAppKeyPressed(wxKeyEvent& evt)
 
 void XS7::OnNewDocLeftClick ( wxMouseEvent& evt )
 {
-  if ( !evt.ShiftDown() )
+  if ( !evt.ShiftDown() || !m_insaneWidget->lblNewDoc->IsEnabled() )
   {
     evt.Skip();
     return;