FlxWeapon: Big Cleanup + Update for HaxeFlixel/flixel#1193

- Removed those `setXXXXX` methods, access the corresponding variables directly instead.
- Use enum for the speed mode of bullets (either constant speed or accelerates)
- Use enum for the origin of firing (either from a fix position or from parent)

At least this compiles for HaxeFlixel/flixel#1193
Will fine tune more in the future.