Support markups interaction with VR controllers

Markups are displayed in the VR view and hovering a controller over a control point highlights it, but they cannot be interacted with: control points cannot be selected, dragged, or placed with the controllers.

Controller input currently only drives prop grabbing and flying; no controller action reaches the markups widgets as a press/release, and the markups widgets themselves only handle 3D move events, not 3D clicks.

Goal: selecting, dragging, and placing control points with either controller, without breaking grip-based object grabbing. This likely needs changes both in the extension (mapping a controller button to a 3D click event) and in Slicer core (3D click handling in the markups widgets).

The VR Stage (MeasurementTool) gives an example of how markups can be created and manipulated in VR using Python, but direct manipulation only using the C++ core would be nicer.

Related: #134, #141