~Reworked entirely the wave system

It took me some time to realise that a "wave system" would be too easy to beat.

I took the decision to take it down entirely and rework it.

Now, it's a game phase of 15 min with more and more enemies spawning as the time passes.

The enemies being also stronger with time.

Not implemented yet but, there will be 3 "boss fights" as the game progresses.

+Timer

It's better for a "time based wave" to know how much time are we in.

The problem is that it does not work at the moment, but hey, it's there!