* Player mode
Add isPlayerOnly property on GUI. Move isFullScreen into mode reducer with isPlayerOnly.
Full screen needs to be a separate prop so that GUI knows what to show when exiting full-screen (either player-only or full editor).
- set `gui-body * {box-sizing: border-box;}` instead of expecting it to be set by the enclosing component/playground, and removed uses of content-box (adjusting sizes as needed)
- Added `!isPlayerOnly` to the conditions for draggable sprites
- rewrote player example to use player mode.