[0.2.8] Endless wave mode

Hi ! This is the biggest update in a long time and I am excited to show you the progress I have made.

Endless wave mode :

This mode is available on the first stage for the moment. It enables a random and procedural generation of each wave and will generate for ever ! Each wave is calculated using parameters such as how many basics turrets can you build with the amount of money you have won so far and is supposed to be intelligently calculated. However, this is a first version and it will certainly have some adjustments in the future. As of now, the waves are generated using my savant calculus and each 10 waves, the enemies gain +100% of their initial HP and armor, to ensure that the waves stay challenging.

An example of the prototype scene I was using for the tests :

Enemies behavior changes :

• Enemies will now pack instead of slowing down. Same enemies type can now overtake each other. This was a major performance hit and gameplay mistake I made a while ago and is now out of the game.
• Enemies will now use the whole road to travel and not just the center of it. This allows to better estimate how many zombies are packed when a large group arrives.

Zombies using the whole road :

Other :

• I simplified a lot of code to make my life a little easier and gain some performances.
• Fixed some UI elements not showing properly and some that could lead to crashes (oops).