Update UI play/stop button when the sound ends

Play button is larger (easier to touch). Can both start and stop sounds.
Sound doesn’t restart on mouseEnter if the sound is already playing.
Sound stops if you move off play button

Missing: stop button does not revert to play button when the sound finishes.

Checked with Carl and updated the style of the play button.

Add prop to library items to show a play button. If a play button is present the ‘play on hover’ only applies over the icon rather than the full tile.

* add scratchLogo as default for logo prop
* prefer `{featureAllowed && (<Component />)} over ternary operator with a null ‘else’
* renamed `handleSaveToComputer` as `getSaveToComputerHandler` to more clearly represent what it’s doing (returning a function to be the handler)
* renamed the parameter that is the function to handle clicking on the load project menu item (also updated the comments in `SBFileUploader`)

Show alternate logo if GUI is initialized with a different file.