Landmarks widget usability improvements

[acardona]

Would be helpful if, in the "Display" tab:

• When choosing a source group, if there is a matching (by name) target group, that the latter is pre-selected automatically if none are selected yet as targets.
• That upon both a source group and a target group both having been selected, a line appears indicating the subset of landmarks that match by name, and further, the subset of those that would yield a PointMatch (they may not if the aren't any associated coordinates with a particular landmark).
• That upon clicking the "Add transformation" button, the number of landmarks used was reported.
• A button to export the transform, e.g., as a list of point matches and the name of the transformation model, or even as a working javascript function that can be used from the console for testing the transform.
• Make the transform editable: for an existing transform, enable ticking/unticking the landmarks that it uses to test how the skeletons would be transformed with/without it. Would help identify badly positioned landmarks. A non-blocking dialog listing the landmarks akin to the one used when editing landmark groups would do.