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/InsaneWidget/CMakeLists.txt

@@ -30,5 +30,4 @@ target_link_libraries(insanewidget minutils stampwidget
                     ${wxWidgets_LIBRARIES}
                     ${LIBINSANE_LIBRARIES}
                     ${PODOFO_LIBRARIES}
-                    ${NETPBM_LIBRARIES}
                     ${PAPER_LIBRARIES})