enh(ui): add contextual mode and menu for fiducials

Description

These changes introduce an editing mode for landmarks and distance. Landmarks and distances are now immutable by default, their respecting editing mode must be on in order to be created, removed, moved and renamed. Now, in order to remove a landmark or a distance, one must click on it while the editing mode is on and then click on the bin button which appears. In order to rename a landmark, one can click on the label and directly edit it. Note that, right now, the landmark groups have names, not individual landmarks; therefore, to rename a landmark will actually modify the name of its group.

Closes #1113 (closed)

How to test it?

Test the creation, removal, move and rename of both landmarks and distances.

You may also launch AddDistance and LandmarksInteraction GUI tests.