Development bugs

* Make fullscreen code editor larger.
* Give camera control buttons fixed positions.
* Fix border issue on disabled buttons.
* User event mixin in LoadingSpinner component, dismiss modal if error occurs.
* Autoformat XML editor content when the component is shown.
* Listen for "enter" keypress in loading dialog, load simulation when it occurs (rather than dismissing the modal)

https://github.com/openworm/org.geppetto/issues/226