[Ready to merge] New features for the thresholding applet

Including, but not limited to:

  * 5d compliance
  * canonical axes order within the operator
  * some bug fixes

Missing:

  - [x] integration tests: single images
  - [x] integration tests: 3d volumes
  - [x] integration tests: time series
  - [x] sciency documentation by @akreshuk 
    -  for the opengm operators `OpGraphCut` and `OpObjectsSegment`
    -  tool tip in `drawer.ui`
  - [x] update viewer on apply click after tab change (see below)
  - [x] removal of duplicate code (as in ilastik/lazyflow#135)
  - [x] Bug: fix #883 
  - [x] serializer not working (`setInSlot` on `OpThresholdTwoLevels`)