Quality of life updates

This update includes many small things that together make the game more pleasant and intuitive.

Gameplay changes:

• added board thickness option.
• menu now defaults to open while playing.

New features:

• middle mouse button to orbit (as an alternative to shift-right-click).
• added "quit" button (in VR menu) 
• added VR locomotion options:
  • grips: the previous system, grab the world and drag it around.
  •  pads: same, but grab the world by pressing down on the joystick (or touchpad depending on controller).
  • sticks: left stick for hand-relative motion, right stick for snap turn.

Bugfixes:

• load button then cancel (in flat) doesn't reset the game anymore.