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/InsaneWorker.h

@@ -88,6 +88,7 @@ private:
   lis_item * m_sourceItem = nullptr;
   InsaneWorkerEvent * m_evh = nullptr;
 
+  bool m_cancelRequested = false;
   std::vector<DeviceDescriptor> m_devices;
   std::string m_deviceId;
   std::string m_source = "FlatBed";