Display thresholding option in Label Viewer

This PR adds an thresholding option to the LabelViewerGUI. I started working on it with @bekaus  a while ago. __It depends on changes/additions in volumina/thresholding__, which now also  contains a branch with the name thresholding. 

An extra button is added next to the eraser in the Training Tab. It also can be enabled by the keyboard shortcut 't'. It only works if the data is grayscale - otherwise it has no functionality. While enabled and the left mouse button is pressed you can, by moving the mouse, adjust the threshold for displaying (the width of the interval (min-max) as well as the position of the interval (mean) within the data range). The values are memorized for each channel separately. Pressing the right mouse button performs an automatic range adjustment. For my kind of image data this feature is indispensable during labeling.