Pass a `stageSize` property down from a centralized `MediaQuery` in the
GUI component, and control the size of the stage and all surrounding
elements based on that value. Note that this is a pure property and is
not stored in the Redux state, since we gain access to the results of
the media query during `render()` and we shouldn't change Redux state
during `render()`.
Some stage-adjacent elements don't yet react correctly to the smallest
stage size.
* 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.
Christopher Willis-Ford