add FlxTypedWeapon

Summary of this PR:

- Added FlxTypedWeapon due to the depreciation of `setAll()`. The replacement `ForEach` requires strictly typed members.
- Moved firing logics from FlxBullet to FlxTypedWeapon.
- Enhanced the bullets creation process. Weapon now calls a custom factory function to create a bullet. (`BulletFactory` replaces `BulletType` in constructor)
- Removed `onFireCallback` and `onFireSound` (they are basically duplicates of `onPostFireCallback` and `onPostFireSound`)

Definitely makes sense to split it into smaller functions... My recent flixel.math pull request should help reduce redundancy (fireAtTouch vs fireAtMouse etc).