Update intensity view widget

This MR mainly changes the widget used to display the scatter plot in the intensity view:

• It replaces the animated toolbar with a toolbar on the top
• It adds a colorbar
• It adds a menu to change scatter symbols and symbol size

It solves the following issues:

• closes #12 (closed): Use same colormap in IntensityView
• closes #20 (closed): Use plot data margins so scatter and curves do not 'touch' the plot edges
• closes #21 (closed): IntensityView: Bottom part takes a lot of space when first shown
• related to #16