Remove submodules

Since we aren't using `FlixelCommunity/flixel-release` anyway, but
isntead switched to using GitHub's "releases" feature.
Re-add Flixel SWCs

No longer submodules. Oh well.
Update code to work with Flixel v2.57-6

Now matches up with commit ca946ab5b2a60ca7d4c1df13e84d29fb2fb40c01

(That is, just after everything has been moved into the `flixel` package
and better organized into subpackages.
Update to Flixel v2.57-6