[Refactor] Rework flixelgdx-core backend to work with flixelgdx-teavm

[stringdotjar]

Currently, the foundation for the framework is solid; however, it needs some tweaks. The flixelgdx-core module (which is used by all other backend platform modules) currently uses classes that aren't supported for TeaVM, such as the java.io.* package (which is for file handling, and browsers handle files very differently than a normal PC would).

This issue requires that the classes that use these blacklisted and unsupported classes to be wrapped behind an interface and injected by a class implementing these interfaces (preferably in the flixelgdx-jvm package, since these classes can be used by normal Java but not the web).