* 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.

This way the instance can be reused, which allows access to non-deprecated `DataFormat` and `AssetType`.

When loading an asset, construct the URL not by using the default data
type of the type of asset being loaded, but the actual data type of the
skeleton asset passed in. This allows the asset requestor to override
the data format, for example when loading a JPEG-format backdrop.

There are a handful of places where the GUI code could use the storage
module directly; converting those is for a later change.