Cancel contextmenu event to stop menu popup on right-click

Firefox shows the context-menu on right-click, even under pointer lock.
(unlike Chrome). Listen to this event and prevent the default action to
prevent right-click block place from showing the menu. Related to #1.
Hide player model in first-person mode. Closes #2

When you looked down in first-person view you would see your body covering
half the screen, so hide it in this perspective. Would still have liked
to show the arms but they hang by your side and were visible too; hiding
everything for now. Remains visible in third-person view.