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/MiscTools.h

@@ -45,6 +45,7 @@ public:
    * Shows a message as a modal dialog or as a notification.
    */
   static void MessageBox ( const wxString& msg, const bool notify = false );
+  static void AsyncMessageBox ( const wxString& msg, const bool notify = false );
 
   /**
    * Creates a validator excluding file name forbidden characters, path