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.

Resources/Utilities/TimeredStatusBar.cpp

@@ -14,7 +14,7 @@ TimeredStatusBar::TimeredStatusBar ( wxWindow * parent)
 {
   m_numFields = 3;
   /* We use default reasonable widths for CAPS and NUM. */
-  const int widths[3] = {-1, 70, 70};
+  const int widths[3] = {-1, 70, 90};
   SetFieldsCount ( m_numFields, widths );
   m_timer.Stop();
   m_timer.SetOwner ( this );