Use key modifiers to handle keyboard events

See http://learningthreejs.com/data/THREEx/docs/THREEx.KeyboardState.html for how to do this. There are properties in the event to indicate if shift, ctrl, alt keys are also pressed. Do not use the threex plugin if I can save myself from adding one more dependency.

Start with the linked THREEx.KeyboardState documentation, then locate the project's existing keyboard event handling. Confirm how modifier-key properties are exposed by the event and verify that the behavior works without adding the THREEx dependency.

Written by the indexing model from the issue text.