chores: Verify if memory leaks are present

[SomePersonFromMars]

In particular, some LibGDX objects require dispose() method to be called in order to free the memory.

[SomePersonFromMars]

• FrogFactory, HedgehogFactory, CoinFactory, BulletFactory implement Singleton using a static construction, therefore its dispose() method is never called, because it is not obvious where a good place is. Perhaps Main should be responsible for constructing such Singletons?

[SomePersonFromMars]

It is probably useful to use AssetManager for centralised assets loading and disposal.

[SomePersonFromMars]

It is generally very hard to verify, whether all Disposable objects are disposed properly. There is no standard LibGDX way.