Simplifying Paint segmentation toolbox

Redmine issue : https://med.inria.fr/redmine/issues/1743

Remove Select/Erase Data : now just click on the image to select it

Remove Boundary function because it is not very useful, and it has bugs.
Remove QTextEdit m_dataText, from what I've seen, it's useless (maybe someone knows what it is for)

Clarify the toolbox user interface by hiding/showing some buttons