Two new missions/locations + NPCs

Hello! This entry of the game corresponds to the last reviewed version within the "Taller Inicial de Videojuegos" course, as the semester comes to an end (2020-2).

Important changes:

- 2 new missions added.

- 2 new locations.

- NPCs corresponding to each of the 4 total missions with dialogue.

- Mission GUI (accessible with "M" key) now displays name of the current zone, instructions for new missions and "Thank you for playing Ant Corp!" message once all tasks are completed.

- Minor updates.

- Bug fixes.