Qspace stack view

This MR adds an image stack view tab to QSpace view + displays the ROI in this widget too (closes #49 (closed) ).

While this was the initial goal, it actually contains more. To make it work, I had to look at how the qspace ROI is selected (closes #30 (closed) ), and I changed the ROI selection (and display of qspace coordinates) from qspace bin centers to qspace bin edges. To me it sounds better, but do you agree with this?

And to make this work I had to update the range slider (and I took this opportunity to refactor it in order to add it to silx).

Still to do: a bit more testing and to implement histogram centers to bin edges at a single place to avoid duplicates.