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.

README.md

@@ -31,6 +31,10 @@ Right click on the 'New' label to specify the number of faces and whether double
 
 Right click on the 'Scan' button to set the device properties.
 
+'Ctrl - right' click on the 'Scan' button to define stamps.
+
+Press 'Esc' to cancel and reset a scan project.
+
 Outputs:
 
  - a single PDF file with the number of requested pages