Implement enemy spawners/generators

[Calinou]

Related to #11.

Enemy spawners should be implemented. Here's how they seem to work in the original game:

• The spawner is made active once the player gets close enough.
• It spawns an enemy of a given type every so often (the delay needs to be measured). It plays a "spitting out" animation while doing so.
  • There is likely a maximum number of alive enemies per spawner too; measure it.
• Dealing damage to the spawner will eventually destroy the spawner.