Crash when quitting while the pipeline is being updated

[mfrischknecht]

Steps to reproduce:

• Create a long lasting pipeline. Example:
  • Load http://www.thehistoryblog.com/wp-content/uploads/2012/03/plan_of_st_gall.jpg
  • Add a median filter with a big window size
• Enable the preview
• While the process is pending, quit the application
• The application will crash "randomly" at a number of different locations in the code
• Perhaps this could be avoided by implementing Provide a possibility to cancel the currently running update process #23.

[schmk2]

Should be fixed by fbb6383
Report back if it still happens.