BREAKING mitk::FeedbackContourTool

Why: Better control over internals. Now the feedback contour has
always the right time geometry (deduced from the working image.
Old code sometimes screwed the feedback contour and/or its time
geometry. Feedback contour is now private and only offers a const
getter. To manipulate the feedback contour new methods are offered:

• InitializeFeedbackContour
• ClearsCurrentFeedbackContour
• UpdateCurrentFeedbackContour
• UpdateFeedbackContour
• AddVertexToCurrentFeedbackContour
• AddVertexToFeedbackContour

How to convert code: use the new function like it is done by the derived
classes provided by MITK directly.

+ improved constness
+ ensured correct handling of time geometry and different time steps.
+ WriteBackFeedbackContourAsSegmentationResult: new convenience function

that eases the correct transfer of the current feedback contour into
the working image with given value/label (including all that stuff like
handling interpolation, undo/redo generation, checking for label set rules...)
Normally classes derived from FeedbackContourTool just have to call this
function when appropriate if the feedback contour represents the segmentation.