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.cpp

@@ -116,6 +116,15 @@ void MiscTools::MessageBox ( const wxString& msg, const bool notify )
   }
 }
 
+void MiscTools::AsyncMessageBox(const wxString& msg, const bool notify)
+{
+  wxTheApp->CallAfter([msg, notify] ()
+    {
+      MiscTools::MessageBox(msg, notify);
+    }
+  );
+}
+
 wxTextValidator* MiscTools::MakeFileNameValidator ( bool excludeSpace )
 {
   wxTextValidator * tval = new wxTextValidator ( wxFILTER_EXCLUDE_CHAR_LIST );