CellTrackingBase: add "merge selected" button, reorder button menus

[smlpt]

This PR adds:

• a new "merge selected" button for the cell tracking interface in manvr3d.
• reorganises the button menus in manvr3d and improves the naming of methods in the 3D cursor tool.
• adds connection checking to the EyeTracking class to allow fallback to the default VR environment
• add a reset view button
• add buttons for deletion of single timepoint or whole graph
• make the scene cleanup after stopping VR more robust