When importing a project we know the file extension for each asset to be
loaded. This change provides that information to the storage system so
that we can load assets which don't use the default. For example, this
allows loading JPG-format backdrops.
In support of this change, there's a new function on `StringUtil` called
`splitFirst`, which splits a string on the first instance of a separator
character. This change includes unit tests for this new function.
* Add costume dataURI to costumes from storage
Towards #515, LLK/scratch-gui#279
* Fix tests
* Load costumes incrementally
Some of our tests assume that at least some of our costume data is available before the costume data is loaded. So, provide as much costume data as is available.
* Remove unnecessary filter for null costumes
Resolved when we updated to load costumes incrementally.
/ht @cwillisf
