Add combo box with different options to display the segmentation result.

TODO: implement the display options.
rename variable to avoid confusions
Fix bug to label dams correctly when using a priority queue.

The dams were keeping INQUEUE value. Now they are initialized to INIT and set to 0 after flooding.
Add implementation of missing display options (overlayed dams and watershed lines).
Redistribute GUI components and divide them into segmentation and display options.
Make sure to display the result with the same slice as the GUI image.