Adding FlxTypedSignal abstract class, inspired by hxsignals.
Consolidating signals implementation into FlxSignal.hx, moving it back to utils package.
Fixing errors and test suite for FlxSignal.
FlxSignal: renaming isOnce to dispatchOnce for clarity.
Moving FlxSignalTest to flixel.util package.
Merge pull request #973 from gamedevsam/dev

Adding FlxTypedSignal abstract class, inspired by hxsignals.