Label sorting with drag and drop

[Tom-TBT]

Hello Will,
here's a PR to sort labels with a drag-and-drop. The cursor changes when hovering the labels, and moving the labels up or down, the order of the labels will be updated.

For the tests I conducted it works well. This handles:

• multiple panels (all labels equal, all different, mix of both)
• multiple label position (one set of positions is updated at a time)

This is the work from my new student Tehmina who is helping me with changes I'd like to make. @MinaEnayat

[will-moore]

PR looks great and works well!

The only question I have is about the destroy() here...
If I create a sortable, then select a different panel in the figure then this SelectedPanelsLabelsView gets lost without destroying the selectable(s) that were created.
I don't know if that really matters - probably not. Does it hurt if we don't clear destroy them? E.g. use up memory etc??
If we do want to make sure they all get destroyed, I wonder if a more global list of sortables (instead of just this component) or possibly try to handle the removal of the component and destroy them there. E.g. see

omero-figure/src/js/views/right_panel_view.js

Line 756 in 6456608

clear: function() {

(which was originally created to handle the destruction of jQuery sliders - now I don't use jQuery sliders, so it does less now).

[Tom-TBT]

I followed the clear example and now destroy the sortables when the panel is deselected. Thanks for spotting that

[will-moore]

Thanks for the fix - testing locally looks good and functional testing on merge-ci is behaving as expected 👍

[will-moore]

This is now released in omero-figure 7.1.0