Implement checkpoints in levels

[benchi99]

Introduce a checkpoint system

• Add a checkpoint flag in GameManager to keep track of current checkpoint status #44
• Add a checkpoint gameobject that takes a checkpoint number value (unique per level, ideally that we can have checkpoint 1, 2, 3... n checkpoints) #45
• Ask artists for art that fits how a checkpoint would look like #46

Open questions:

• Do we want to have a sound effect for grabbing a checkpoint?
• Do we want to store position of UFO & alien independently? (I.E. Adding a checkpoint in level 3 where they're both split most of the time)