reworked ThresholdTwoLevels (3rd try)
Counting: fixing some (hopefully all?) causes of segfaults due to boxes
change output to uint32 everywhere
fixes #763
StandardApplet: Fixed call to base class constructor (interactive parameter)
Fixes #846
AppletDrawerToolBox: If the client attempts to select an invisible item, just ignore it (dont' assert).
Merge branch 'pr-thresholding-rewrite3' into pr-thresholding-rewrite4

Conflicts:
	ilastik/applets/thresholdTwoLevels/opThresholdTwoLevels.py
ObjectExtraction: The non-interactive gui is now a lightweight stand-in (no layer viewer, etc.)
Merge pull request #848 from burgerdev/pr-thresholding-rewrite4

rewrite of ThresholdTwoLevels (rebased to current master)
Labeling Applets: When responding to changes in label colors, ensure that the layer still exists in the stack before changing the layer color.
(It's too bad we have to do this.  We need to find a cleaner fix than this.)